THE USE OF A-FLUTE, B-FLUTE, AC-FLUTE, AND BC-FLUTE CORRUGATED PAPERBOARD AS A CUSHIONING MATERIAL

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1 Clemson University TigerPrints All Theses Theses THE USE OF A-FLUTE, B-FLUTE, AC-FLUTE, AND BC-FLUTE CORRUGATED PAPERBOARD AS A CUSHIONING MATERIAL Alicia Campbell Clemson University, tigerlily_lkc@yahoo.com Follow this and additional works at: Part of the Engineering Commons Recommended Citation Campbell, Alicia, "THE USE OF A-FLUTE, B-FLUTE, AC-FLUTE, AND BC-FLUTE CORRUGATED PAPERBOARD AS A CUSHIONING MATERIAL" (2010). All Theses This Thesis is brought to you for free and open access by the Theses at TigerPrints. It has been accepted for inclusion in All Theses by an authorized administrator of TigerPrints. For more information, please contact kokeefe@clemson.edu.

2 THE USE OF A-FLUTE, B-FLUTE, AC-FLUTE, AND BC-FLUTE CORRUGATED PAPERBOARD AS A CUSHIONING MATERIAL A Thesis Presented to the Graduate School of Clemson University In Partial Fulfillment of the Requirements for the Degree Master of Science Packaging Science by Alicia Cutler Campbell August 2010 Accepted by: Dr. Duncan O. Darby, Committee Chair Mr. Gregory Batt Dr. Matthew Daum i

3 ABSTRACT Corrugated paperboard, often referred to as cardboard by those outside the industry, is most often associated with the typical shipping box. Research has proven that cushioning is another possible packaging application for corrugated paperboard. Corrugate is easily accessed, obtained from a renewable resource, and easily recycled all of which appeal to the increased interest in the use of sustainable materials. Most research related to corrugate as a cushioning material has been performed using single-wall, C-flute corrugated paperboard. The purpose of this research was to create cushion curves for A-flute, B-flute, AC-flute, and BC-flute corrugated paperboard using the stress energy method. Cushion curves were created and presented by the common industry practice of drop one and the average of drops two - five. However, it was determined that cushion curves for corrugate materials should likely be presented as drop one, drop two, and the average of drops three-five due to large differences between drops two and three-five. The stress-energy method was used to create the cushion curves, which required the use of a trendline to characterize the material. The type of trendline applied to the dynamic stress vs. dynamic energy graph was examined. Linear, exponential, third order polynomial and fifth order polynomial trendlines were applied to all flutes and conditions. Exponential line fit showed comparable r-square values, especially after drop one. However, for drop one, a polynomial line-fit is likely still required due to lower correlation coefficient (r-squared) values. ii

4 The homogeneity of lines for all flute sizes was compared using an exponential line fit. All flute sizes were determined to be statistically different from one another. A-flute, B-flute, and BC-flute cushions were tested at both standard and high humidity conditions to determine if there was a statistical difference in the cushion curves. All flute sizes conditioned at high humidity conditions were determined to be statistically different from the same flute size at standard conditions. Also, all flute sizes showed lower cushion effectiveness under high humidity after multiple drops. iii

5 DEDICATION I dedicate this work to my mother who continually encourages me and always pushes me to achieve my best. iv

6 ACKNOWLEDGMENTS I would like to thank my husband and my family for their love and support during my graduate studies. I would especially like to thank my brother, Kyle, for his assistance with the testing portion of my research. I would like to thank my advisors for the guidance and support during my research and the writing of my thesis. I greatly appreciate the assistance of Jerry Stoner, Jason Symanski, and Glen Potter with the changing out of weights on the cushion tester. This enabled me to test my samples in a timely manner. I also would like to thank Filip and Carson for aiding me in the drawing and cutting of the test samples. Also, I would like to thank International Paper for the material needed for my research. v

7 TABLE OF CONTENTS TITLE PAGE... i ABSTRACT... ii DEDICATION... iii ACKNOWLEDGMENTS... vi LIST OF TABLES... vii LIST OF FIGURES... viii CHAPTER I. INTRODUCTION... 1 II. REVIEW OF LITERATURE... 5 Corrugated Paperboard as a Material... 5 Cushioning Corrugate as a Cushioning Material Cushion Testing Cushion Testing of Corrugated Paperboard III. MATERIALS AND METHODS Materials Basis Weight Determination Construction of Corrugate Cushion Samples Initial Test Plan Testing Statistical Analysis IV. RESULTS AND DISCUSSION Basis Weight Summary of Materials Tested Results from Testing A-Flute at Standard Conditions Dynamic Stress vs. Dynamic Energy Different Line Fit Methods Page vi

8 Statistical Analysis of Trend lines Cushion Curves for A-flute Actual Acceleration (G s) vs. Predicted Acceleration (G s) Summary of B-flute, BC-flute, AC-flute Humidity Effects Recommendations for Future Research V. CONCLUSIONS APPENDICES A: Energy Levels Tested B: Dynamic Stress vs. Dynamic Energy Equations C: SAS Code for Statistical Analysis D: Actual versus Predicted Acceleration Values REFERENCES vii

9 LIST OF TABLES Table Page 1 Mean Basis Weight Values and Standard Deviations for A-flute, B-flute, AC-flute, and BC-flute material used for research Dynamic Energy Levels, including test parameters, tested for A-flute at Standard Conditions An example set of data collected for a selected dynamic energy level for A-flute Dynamic Stress vs. Dynamic Energy Equations for A-flute at Standard Conditions, Drops 1-5 and Average of Drops 2-5 (Polynomial) R-square values for A-flute at Standard Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines R-square values for A-flute at High Temperature, High Humidity Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines R-square values for B-flute at Standard Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines R-square values for B-flute at High Temperature, High Humidity Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines R-square values for BC-flute at Standard Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines R-square values for BC-flute at High Temperature, High Humidity Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines viii

10 List of Tables (Continued) Table Page 11 R-square values for AC-flute at Standard Conditions Using Exponential, Linear, and Polynomial (3 rd and 5 th order) Trend lines Dynamic Stress vs. Dynamic Energy Equations for A-flute at Standard Conditions, Drops 1-5 (Exponential) Homogeneity of Lines: Difference in Slopes at Standard Conditions Homogeneity of Lines: Difference in Intercepts at Standard Conditions Homogeneity of Lines: Difference in Slopes at High Humidity Conditions Homogeneity of Lines: Difference in Intercepts at High Humidity Conditions Actual Acceleration values (G s) vs. Predicted Acceleration values (G s) for A-flute, Drop Actual Acceleration values (G s) vs. Predicted Acceleration values (G s) for A-flute, Drop Homogeneity of Lines: P-values for High Humidity vs. Standard Conditions for each flute type (A-flute, B-flute, & BC-flute) Homogeneity of Lines: P-values for Comparing Multiple Flute Sizes at Standard and High Humidity Conditions ix

11 LIST OF FIGURES Figure Page 1 Diagram of Corrugated Fiberboard Construction Diagram of Double-wall Corrugated Paperboard Comparison of Common Flute Sizes Shock pulse for both a Cushioned Response and Non-Cushioned Response A typical cushion curve Areas of interest in a typical Cushion Curve Cushion Tester Set-up Image of Corrugated Paperboard in both Edge Crush and Flat Crush Orientations Diagram of Mis-aligned, Perpendicular, and Aligned flute Corrugate Cushion Designs A cushion created with the combination of a crumple element (virgin corrugate) and a primary element (pre-compressed corrugate) Image of the Machine Direction of Corrugated Paperboard Image of Cushion Samples before testing Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Drops 1-5 (Polynomial) Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Drop 1 (Polynomial) Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Avg. Drops 2-5 (Polynomial) x

12 List of Figures (Continued) Figure Page 16 Dynamic Stress vs. Dynamic Energy for C-flute at Standard Conditions, Drop Dynamic Stress vs. Dynamic Energy for B-flute at Standard Conditions, Drop 1 (Polynomial) Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Drops 1, 2, 3, 4, 5 (Exponential) The r-squared values for A-flute, B-flute, BC-flute, and AC-flute with fifth order polynomial, third order polynomial, Exponential, and linear line fit Scatter plot generated by SAS for the lx and ly of A-flute and B-flute Cushion curve for A-flute at standard conditions Cushion Curve for A-flute Corrugate at High Humidity Conditions xi

13 CHAPTER ONE INTRODUCTION The need to transport items from one location to another has existed since ancient times. Transported items often require protection and/or packaging in order to get from one place to another without being damaged. Transportation has only increased since ancient times and in today s world, items are being shipped farther and farther distances. The U.S. relies heavily on safe transport of items from places such as China and other foreign countries. These extreme distances make transport expensive. The more options a packaging designer has in order to safely protect the product, the more likely the packaging engineer will be successful at reducing some of the overall cost. The main modes of transportation are trucks, ships, planes, and trains which all bring about various potential hazards that must be considered. The three main hazards that occur during the transport of packages are shock (impacts), vibration, and compression. Damage can be caused by any of the three or a combination of two or more. It is important that a packaging engineer knows the mode of transport so that the likelihood of these hazards can be predicted. It is also important to determine the potential drop height the packaged product may encounter and the fragility of the product itself, also known as the critical acceleration. The first of the three potential hazards that may be encountered is shock. Shock occurs when there is a sudden increase in velocity (falling) followed by a sudden decrease in velocity (hitting a surface). Shock can occur in a range of distribution 1

14 environments such as manual handling, falling from a forklift or storage area, a sudden stop by a truck or train, and in many other situations. The second of the three potential hazards is vibration. Vibration is oscillating motion over time. A package will endure vibration when it is transported by truck, rail, or plane. Vibration is often over looked when considering potential hazards but it can be as damaging as both shock and compression. The third of the three potential hazards is compression. Compression occurs when a pushing force reduces the volume of an object. Static compression is a loading force that a package will endure when it is stacked vertically for an amount of time. The force being applied is not moving. A package may also encounter dynamic compression while being transported in the back of a truck. Dynamic compression occurs when there is a moving force being pressed against the object. Dynamic compression is the form of compression that is observed during cushion testing. Cushioning is one element of a packaging system that can reduce the effects of shock, vibration, and compression in the transportation environment. Cushions come in a variety of forms, which include packaging peanuts, polymeric bags filled with air, foambased cushions, molded pulp, and corrugated cushioning. Corrugated cushioning offers a more environmentally friendly alternative to the commonly used polymer-based foam cushions. Corrugated paperboard is derived from cellulose, which comes from renewable resources. Corrugated paperboard can easily be compressed and baled for transport to recycling facilities. There are numerous recycling programs already in place. It was estimated that in 2007, over 70% of corrugated 2

15 paperboard that was produced was collected and recycled (Corrugated Packaging Alliance, 2010). Also, corrugated paperboard is mass-produced so it is readily available. This makes it easy for the packaging designer to obtain and use corrugated board as a cushioning material if desired. The expected behavior or capabilities of a cushion can be predicted using cushion curves. Cushion curves are generated using either ASTM Standard D1596 (ASTM, 1997) or ASTM Standard D4168 (ASTM, 1995). These standards require many hours of work and thousands of drops in order to generate cushion curves. Also, these cushion curves are specific to a given material, drop height, static load, and thickness. The more recently developed stress energy method can greatly reduce the amount of time needed in order to create cushion curves for a given material. The stress energy method uses dynamic stress and dynamic energy to characterize the cushion material so that cushion curves can be produced for a variety of thicknesses and static loads using the data collected from a limited number of energy levels. Past research has proven that the stress energy method can be applied to corrugated paperboard (Wenger, 1994). Prior to that research, it was thought that the stress energy method was only effective for closed-cell foams. Most previous research of corrugated as a cushioning material has been performed on C-flute. There has been littleto-no research performed using the stress energy method on A-flute, B-flute, and doublewall corrugated paperboard. It is hypothesized that the variation in flute size and the combination of flute sizes within a cushion will likely prove to have different results than 3

16 those of C-flute. Also, corrugate is known to be moisture sensitive so it is hypothesized that moisture will have an effect on the cushioning ability of the corrugate that is tested. The detailed objectives of this study are: 1. Use the stress energy method to construct cushion curves for A, B, AC, and BC flute corrugate cushions 2. Examine the line-fit method used to generate the stress energy equation for corrugated cushioning materials 3. If an exponential fit was considered acceptable, determine if there is a significant difference in the cushioning abilities of corrugate across the tested flute sizes (A-flute, B-flute, BC-flute, and AC-flute) 4. If an exponential fit was considered acceptable, determine if there is a significant difference in the cushioning abilities of corrugate at standard conditions versus high humidity and high temperature conditions (A, B, and BC flute corrugate) **AC-corrugate was not tested for humidity effects but was used in other areas of this research. 4

17 CHAPTER TWO REVIEW OF LITERATURE Corrugated Paperboard as a Material Corrugated paperboard can be referred to by many different terms such as corrugate, corrugated, and corrugated fibreboard. Those outside the industry also often refer corrugated paperboard to as cardboard. Corrugate is most often associated with the typical shipping box or shipper and is a $25 billion a year industry. A shipper is the informal jargon used to describe a corrugated box or a corrugated shipping container (Selke & Twede, 2005). The regular slotted container, or RSC, is the most commonly used shipper (Soroka, 2002). Although corrugated paperboard is often used as a shipper, it has many other possible applications as well, such as cushioning, point-of-purchase displays, and even works of art. It is an appealing material because it has good strength and is easily recycled. Corrugated is constructed of paperboard which is obtained from cellulose fibers found in wood (Richmond, 2005). The wood used comes from commercially grown trees, so corrugated is derived from a renewable resource. Corrugated paperboard is built using containerboard in two different configurations as seen in Figure 1. These two configurations include linerboard (flat strength portion) and the medium (flutes). Containerboard is a grade of paperboard designed specifically for corrugated board. Containerboard can also be referred to as corrugated case medium or CCM. 5

18 Figure 1. Diagram of Corrugated Fiberboard Construction Flutes are the arches that are contained between the two linerboards. The flutes act like arches, resisting flat crush, to provide cushioning trapped air, and give some insulation properties (Selke & Twede, 2005). These arches offer strength to the corrugated structure. The linerboard is typically constructed using unbleached kraft because of the strength of its fibers (Selke & Twede, 2005). Kraft paper is named after the German meaning of the word kraft meaning powerful or strong. Kraft paper is produced using the kraft process. This process uses chemicals, such as sodium hydroxide and sodium sulfide, to break down wood into wood pulp by breaking down the lignin. The lignin is the sticky substance that holds the fibers together. The wood pulp contains the cellulose fibers needed to produce the paper. Kraft paper inherently has a rough texture due to the fibers. The corrugated medium typically requires less strength than the linerboard so recycled corrugated materials would be acceptable. The fibers are broken down with every recycling so each time the board is recycled it loses some of its strength. This loss of strength allows the recycled paper to be more easily formed into the fluted formation. Most often, two liners and one fluted medium are combined to form what is known as single-wall corrugated board (Soroka, 2002). There can also be the combination of only one linerboard and one fluted medium to form single-face 6

19 corrugated board, which is flexible in one axis (Soroka, 2002). Double-wall corrugate contains three linerboards and two layers of fluted medium as seen in Figure 2. Figure 2. Diagram of Double-wall Corrugated Paperboard The fluted medium of the double-wall corrugate may be the same flute size or it may be a combination of flute sizes depending on the application. Multiple walls, such as double wall, are used when puncture resistance are a concern. The linerboard and the medium are typically combined using a starch-based adhesive. This adhesive is fairly inexpensive and it allows the corrugated paperboard to be easily recycled. However, starch-based adhesive can also present issues because it loses strength when exposed to moisture. Corrugated is classified by its flute size. Common US flute sizes and their respective flute width measured from peak-to-peak include A-flute (.184 inches), B-Flute (0.097 inches), C-flute (.142 inches), E-flute (.047 inches), and Micro-flute ( inches). It is also very common to classify flutes based on the number of flutes per foot (Selke & Twede, 2005). These values include A-flute (33 ± 3), B-flute (47 ± 3), and C- flute (39 ± 3) (Schueneman, 2010). Some of the more common flute sizes are shown in Figure 3. 7

20 Figure 3. Comparison of Common Flute sizes (Chris 73, 2010) This Wikipedia and Wikimedia Commons image is from the user Chris 73 and is freely available at under the creative commons cc-by-sa 2.5 license. There will be slight variations from these specifications from manufacturer to manufacturer due to variations in the manufacturing process, but these are the typical measurements. Single-wall boards with B- or C-flute are the most commonly used corrugated paperboard sizes with 70% of all corrugate made in the US being C-flute. (Schueneman, 2010) Within flute sizes, the basis weight of the linerboard and the fluted medium can vary. Basis weight is defined as the weight in pounds per 1000 square feet. The packaging engineer, to get the product qualities needed, may vary the linerboard weights and medium weights. A-flute corrugate has been used as a cushioning material for many years because it has one of the thickest profiles, which allows for good flexibility and compression strength. Thinner flute profiles, such as B-flute, will allow for better printability but less flexibility and compression strength. C-flute was created in order to capture the best of both worlds by designing a profile that is between that of A- and B-flute. The intent was 8

21 to offer a corrugate grade that can be printed easily as well as offering good compression strength (Selke & Twede, 2005). In the packaging industry, the largest application for corrugated is the regular slotted container (RSC), which is known to those outside the industry as a cardboard box. Internationally, a regular slotted container is referred to by the 4-digit code Corrugate is also used to make trays and point-of-purchase displays. Currently in the marketplace, paper is typically used as a loose-fill type of cushion and corrugate can sometimes be seen in brace form for heavier items such as furniture. In recent years, there has been some research into the manufacturing of pallets designed strictly from corrugated (Masood & Haider, 2006). Corrugated cushions have the potential to be used to cushion heavier products such as printers, furniture, copiers, and other heavy electronic items. It may also be applicable to smaller, more lightweight products in place of the more commonly used polymeric foam cushions. The properties for corrugated paperboard will vary based on the size of the flutes, the amount of moisture present, the rate at which the load is applied, the recycled content, and the basis weight. Effects of Moisture on Corrugate Water has many detrimental effects on the properties of corrugated. It creates an overall weakening of the fibers, which in turn reduces the overall strength of the corrugate (Selke & Twede, 2005). Corrugated board loses about 50 percent of its compression strength when it is exposed to at or above 50 percent relative humidity 9

22 (Soroka, 2002). Also, the starch-based adhesive used to hold the linerboard and corrugated medium together does not tolerate high levels of moisture. The adhesive loses its strength when exposed to high humidity conditions (Soroka, 2002). Expansion occurs when corrugated takes on water, and it shrinks when it is dried. There can be as much as a 0.8% change in the MD and a 1.6% change in the CD between 0 and 90% R.H. (Soroka, 2002). After the corrugated has taken on moisture, warpage will occur if the material does not dry evenly. The use of resins and wax can be used to improve water resistance but will also greatly increase costs and may impact its recyclability. However, resins have not shown to greatly improve strength at normal lab conditions (50% and 23F) but it does greatly improve strength when moisture is present (Soroka, 2002). Also, there have been many advancements in water resistant adhesives and treatments that do not cause problems in the recycling process so recyclability may not need to be considered when considering the use of resins (Selke & Twede, 2005). The Effect of Recycled Content in Corrugate In the United States, a corrugate board can still be considered virgin board even if it contains some portion of recycled content (Selke & Twede, 2005). Although even if it is a very small amount, it could still play a role in the rigidity of the board. Since very few companies, if any, use 100% new material when producing corrugate, it is important to know how the recycled content will affect the properties of corrugate. Recycled content has both positive and negative effects on corrugated paperboard. The positive effects of recycling fiberboard included increased smoothness of the linerboard surface, 10

23 improved porosity, and improved printability. Porosity refers to the quality of paper that allows air and moisture to pass through. With recycling, the fibers are shortened which allows them to be more tightly packed. In turn, the porosity is decreased. The negative effects include the corrugate s ability to resist outside forces such as puncture and tearing. However, the recycled content does not appear to have either a negative or a positive effect on the stiffness of the board (Schueneman, 2010). If the stiffness is not affected, then in theory it should have little effect on the cushioning properties of corrugate at standard conditions. However, if it allows the absorption of more moisture, there may be more of an effect on the cushioning properties at higher humidities because of loss of overall strength. The amount of recycled content used will depend on the qualities desired of the corrugated material. When the recycled content is stated for a particular board, it is often based on an average. It would be uneconomical for a company to measure the recycled content of every single board that is produced, so they typically take a monthly average (Selke & Twede, 2005). It is common practice to use at least some recycled content in the manufacturing of corrugated board, but it is difficult to know exactly how much recycled content is in any given sample. Basis Weight Basis weight is determined by the weight in pounds per 1000 square feet (Soroka, 2002). The most common basis weight combination is to use a 42 pounds per thousand square feet linerboard with a 26 pounds per thousand square feet medium (Selke & 11

24 Twede, 2005). Although these are the most common constituent weights, there are many other combinations that can be created and used, based on the intended application. Basis weight can play a large role in stacking strength, flexibility, and ultimately cushioning properties. Higher basis weights result in stiffer and stronger board. This could potentially equate to a much higher shock value to the packaged product due to a lack of spring like qualities. Shock, as mentioned before, is a sudden change in velocity and, in this case, a sudden change in velocity that the package and packaged product encounters (Harris, 1988). A stronger (higher basis weight) board will likely have a higher resistance to force, which will result in higher deceleration values. This would cause more damage to the product because the corrugate would not cushion the product. The cushion would not deflect; therefore the product would experience higher acceleration values. The basis weights for the materials used in this research were tested using TIP This is a standard produced by the Technical Association of the Pulp and Paper Industry to allow engineers to determine the basis weight of a board. The samples are cut and conditioned. The samples are cut to a size of square inches using one of several length and width combinations. After conditioning at standard lab conditions (50% RH, 23F), the sample is weighed and multiplied by 10 to determine the weight per 1000 square foot. This results in the basis weight of both the linerboard and flutes combined. The layers can be carefully separated, if needed, to determine the basis weight of each the linerboards and the fluted medium. 12

25 Mullen Burst Test The Mullen Burst Test is one of the oldest and most commonly used tests to characterize a sheet of corrugated. The test method is TAPPI T810: Bursting Strength of Corrugated and Solid Fiberboard. In spite of its long-term status and popularity, the usefulness of Mullen Burst results has been questioned (Selke & Twede, 2005). The usefulness of the test will greatly depend upon the intended application because Mullen burst values more accurately describe puncture resistance as opposed to overall strength of the board. The test is performed by placing a sample of the corrugate between two platens with a hole in the center. The lower platen has a rubber membrane that will expand and push toward the corrugate as air pressure is applied. Eventually the board will burst and a Mullen burst value will be recorded. The value that is recorded is measured in pounds per square inch and is most closely related to the strength of the linerboard. It can be related to a certain basis weight of the liners based on historical data but it does not give an idea of the overall strength of the board on its own (Selke & Twede, 2005). Flat Crush Test The flat crush test measures the strength of the flutes for corrugated paperboard. There is a constant pressure placed on the flute to see how much force they can resist. The standard for the flat crush test is designated by TAPPI T 825 om-03. This standard is suitable only for single wall corrugated paperboard. The corrugated paperboard is subjected to a constant deflection rate while placed in a rigid support (TAPPI, 2003). 13

26 The values collected from the flat crush test can be used to characterize a corrugated material that is used for the cushioning. The results is thought to give some indication of how stiff the flutes are and how the corrugated may react as a cushioning material. A material that is too stiff may result in poor cushion properties because the deflection will be too small, meaning that high acceleration values will be experienced by the packaged product. A material that is not stiff enough may quickly bottom out which will also give high acceleration values. It is important that a cushion is neither too stiff nor too pliant. If the cushion is too stiff, then there is little to no deflection and the impact energy is then passed on to the packaged product. If the cushion is too pliant, then there is a large amount of deflection and again the impact energy is passed on the packaged product. The focus of this research was the use of corrugated as a cushioning material. To better understand the basis of this research, it is important to understand the role that cushioning plays in the packaging system (the combination of the package and the product). Cushioning Cushioning plays a very important role in distribution packaging. It helps protect the product from hazards such as shock, vibration, and compression. A cushion will aid in the controlled, gradual slow down of the product when it encounters an impact. The gradual slow down is a result of the deflection of the cushion. All cushion systems work in the same way; they trade a high peak short duration shock pulse for a longer duration 14

27 lower peak shock pulse (Schueneman, 1993). A shock pulse is a measurement of the change in acceleration over time as seen in Figure 4. The energy in the system is neither gained nor lost because the area under the curve does not change. The cushion just extends the length of the shock pulse and decreases the peak acceleration observed. Figure 4. Shock pulse for both a Cushioned Response and Non-Cushioned Response The stiffness of a cushion will effect the acceleration values that the product experiences. Stiffness is related to acceleration values in Equation 1 where Gm = the maximum acceleration (in g s), h=drop height of the drop platen, W=the weight of the falling object and k = the linear spring constant of the cushion. Equation 1. Gm= 2kh W 15

28 According to the equation, as stiffness increases, so do the acceleration values experienced by the packaged product. However, the weight of the product also has to be taken into account. A stiffer cushion is needed for a heavier product to prevent bottoming out so increased stiffness does not necessarily mean decreased cushioning ability. Cushions are typically evaluated by ASTM D1596 (ASTM, 2002) or ASTM D4168 (ASTM, 2006), which gives the data collection procedure needed for a packaging designer to create a useful tool known as a cushion curve. An example of a typical cushion curve can be seen in Figure 5. Acceleration Figure 5. A typical Cushion Curve (Nova Chemical 2007) 16

29 The proper cushion is selected so that it protects the product without over-packaging. Over-packaging, with regards to cushioning, means that the area of the cushioning is too big. This increases stiffness leaving little-to-no deflection from the cushion when the package experiences a shock. By over-packaging, the engineer is likely spending too much on packaging while the product is still experiencing the negative effects of the shocks that it encounters. It is important for the packaging engineer to know the distribution system in order to select the proper cushion. Cushion selection requires the knowledge of likely hazards the package will encounter in the distribution cycle. The hazards must be known in order to design the package and packaging components so that they protect the product against those hazards encountered. Cushion selection is also based on how fragile the product is, how heavy the product is, and the information provided by cushion curves. The packaging designer wants to maintain the lowest cost possible for the packaging system to protect the product in the distribution environment. The packaging system includes all packaging components as well as the product. The most cost effective protection may include modifying the product so that it is less fragile rather than adding more protective package. However, unless the product and packaging were designed simultaneously, this may not be a feasible option. The addition of a cushion to the packaging system is a good option to offer the protection needed to get the product to its destination undamaged. Cushion curves are used to select the proper cushion for the given application. 17

30 Cushion curves created for use by packaging engineers are specific to a material, thickness, and static load. The static load is related to the product weight distribution. Static load is defined as weight of the product divided by area where the product comes in contact with the cushion. To create a cushion curve, a material and a density of that material are selected. A range of drop heights and static loads are selected for multiple different thicknesses of the given material. The drop height is a characteristic of the distribution cycle. A package and the packaged product are likely to encounter certain drop heights based on their combined size and weight. However, on a cushion tester, the impact velocity is measured and used to calculate the drop height using Equation 2. The value calculated using Equation 2 is known as the equivalent free fall drop height (h eq ). As seen in Equation 2, the impact velocity (v i ) and acceleration due to gravity (g) are needed in order to calculate the equivalent free fall drop height. The equivalent free fall drop height must be used because the platen slides on rods and friction must be taken into account. Equation 2. h eq = v i 2g The cushion tester is used to collect acceleration values for the selected range of drop heights, static loads, and thicknesses. It is important to note that the acceleration values that are recorded are actually deceleration values. They are deceleration values because the weighted drop platen actually comes to a stop and then rebounds and moves 18

31 in the opposite direction from the initial fall. The terms acceleration and deceleration are often used interchangeably in the industry when referring to the values recorded from the cushion tester. From this data, the packaging engineer can determine if this material will work for the packaged product. The test method for generating a set of cushion curves requires many drop heights, static loads, cushion thicknesses, and appropriate replications so many hours of lab time are required. Figure 6. Areas of interest in a typical Cushion Curve 19

32 When examining a cushion curve, like that in Figure 6, there are three areas of interest. The left most portion of the graph, Zone 1, is an area of high acceleration values. If the product being packaged falls in this range, it is likely to experience high acceleration values because the static stress is not high enough to deflect the material. In other words, the product is not well protected because the cushion is too stiff for the load. The center portion of the graph, Zone 2, is the optimum area to design within because the force is great enough to deflect the material and therefore transform a quick, strong impact into a more manageable, longer impact. The right portion of the graph, Zone 3, where the acceleration values starts to increase again, is when the force is too great for the cushion. In this case, the cushion is bottomed out. Bottoming out means that the cushion is fully compressed. Full compression occurs when the there is no longer any air trapped within the cushion. In the case of corrugate, the flutes are completely crushed and all linerboards are touching. If the cushion is fully compressed, then the force will be transmitted to the product because the cushion has deflected to its maximum without significantly reducing the maximum acceleration. The cushion has lost its resilience and has compacted. Since the cushion has compacted, the stiffness has increased which translates into higher acceleration values. The drop platen could also hit the base of the cushion tester causing the accelerometer to detect very high acceleration values. Recent interest in sustainability has increased the interest in corrugate as a cushioning material. Corrugate could serve as an alternative source of cushioning material. 20

33 Corrugate as a Cushioning Material As previously mentioned, corrugate is constructed of at least one linerboard and one layer of fluted medium. The linerboard and flute combination offers both strength and resilience. It is built to withstand force. This resistance to outside forces makes some skeptical of corrugated paperboard s cushioning abilities. However, there are hundreds of different variations of corrugated due to various board types, basis weights, flute sizes, recycled contents, adhesive types, and coatings (Shin, 2004). Therefore, there is likely some form of corrugate that could serve as an alternative to the more commonly used polymer based cushions. The most commonly tested corrugated material for cushioning is a single-wall C- flute. The basis weight has likely varied between researchers because it has not been defined in the written research. However, it is possible that the basis weight may play a significant role in the cushioning ability of corrugate because it has a great effect on the stiffness of the board. In order to determine if different variations of corrugate will serve as an acceptable cushioning material, it will be subjected to test on the cushion tester just as polymeric cushions are. Previous research by Wenger (1994) proved that the cushioning abilities of C-flute corrugate could be characterized by the stress-energy method. Wenger s finding served as the basis for this research. 21

34 Cushion Testing ASTM D1596 (ASTM, 1997) utilizes a cushion tester that consists of a variety of weights that will be dropped on a cushioning material on a guided platen. The cushion tester used for this research can be seen in Figure 7. Figure 7. Cushion Tester Set-up Based on the material and density being tested, a range of thicknesses, drop heights, and static loads are selected and a cushion curve is generated. The values are specific to the thickness and static load values selected. A curve set can require many hours of lab time. However, an alternative data presentation method, known as the 22

35 stress-energy method, has been developed to try to address these issues and offer a faster way to characterize a cushion (Burgess, 1990). Stress-Energy Method The stress-energy method saves the researcher many hours of lab time while also characterizing the material that is being tested. The characterization of the material allows the researcher or the packaging designer to determine the cushioning ability of the corrugated board over a range of drop heights, number of drops, thicknesses, and static loads. This is of great value to the designer because the cushion curves are not specific to a small range of variables, which offers more design options. The stress-energy method was developed by Burgess (1990). It was first developed strictly for use with closed-cell foams but has since been used to determine the cushioning abilities of materials such as corrugated board (Wenger, 1994). Like ASTM D1596 (ASTM, 1997), the stress energy method characterizes the material based on the relationship between the static load (s), drop height (h), cushion thickness (t) and acceleration values (G). However, the stress-energy method processes the data differently. The dynamic stress (G*s) vs. dynamic energy (sh/t) is plotted and a trend line is fitted to the data. For polymeric foams, this line will describe the cushion behavior through and exponential line fit and the following equation: Equation 3. y=ae bx 23

36 The dynamic stress (G*s) is plotted on the y-axis and dynamic energy on the y-axis. The a and b values are dimensionless constants that are unique to the material that was tested. The a and b values can be used to generate any cushion curve for the material and density that was tested. The data collection for the stress-energy method and the more commonly used ASTM D1596 (ASTM, 1997) is the same. However, the stress-energy method takes the data collected and creates an equation that can be used to create any cushion curve for a given material within that material s limitations. Also, the stress-energy method has been shown to produce cushion curves for polymeric foams with significantly less data collection (Marcondes et al, 2008). Research performed at Clemson University has shown that through statistical analysis the number of energy levels needed to create a cushion curve can be reduced to just three energy levels in order to characterize a polymeric closed cell foam s cushioning abilities (Marcondes et al, 2007). However, it has not been determined if this can be applied to corrugated cushions. Cushion Testing of Corrugated Paperboard Use of Stress-Energy Method with Corrugated Cushions When testing corrugated samples for cushioning properties, there are two common orientations in which the corrugate can be presented. Those orientations include both the flat crush and the edge crush orientation as seen in Figure 8. 24

37 Edge Crush Orientation Flat Crush Orientation Figure 8. Image of Corrugated Paperboard in both Edge Crush and Flat Crush Orientations In edge crush orientation, the flutes are parallel to the force being applied. The acceleration values for these orientations are different from one another. The flat crush orientation produces lower acceleration values, however it can withstand less weight and a fewer amount of drops/impacts as compared to the edge crush orientation. The edge crush orientation will give higher acceleration values, however it can withstand a greater amount of weight and a greater amount of drops. The use of either orientation will depend on the application or its intended use. The edge crush orientation will likely offer good protection if the product is heavy and the cushion is designed for bracing. When a material is used for blocking or bracing, no deflection is desired. A block or brace is designed to fill space or hold a product in place. Whereas, with a cushion, it is important that the material deflect in order to protect the product. It has been determined through the testing of C-flute corrugated cushions that the stress-energy method can be applied to the production of cushion curves for corrugated materials. It was previously thought that the stress-energy method might not be 25

38 applicable to corrugate because it is not a closed-cell cushion. The exponential line fit used for closed cell foams has a basis in the theory of compressing a sphere (Daum, 2010). However, there is no convenient theoretical basis for corrugated cushioning. In spite of this, Wenger (1994) found that an empirical fit using a fifth degree polynomial was sufficient to allow the stress-energy approach to work for corrugated cushioning. The Effect of Changing the Alignment of the Corrugate Cushion Shin (2004) investigated whether or not the alignment of the flutes, when creating a cushion out of corrugate, played a role in its cushioning abilities. Three different combinations were tested including: mis-aligned, perpendicular, and aligned flutes as shown in Figure 9. mis-aligned flutes perpendicular flutes aligned flutes Figure 9. Diagram of mis-aligned, perpendicular, and aligned flute corrugate cushion designs (Shin, 2004) **Permission requested, awaiting reply Research determined that these various alignments played little, if any, role in the cushioning ability of a corrugated cushion. This conclusion makes the assembly of corrugate cushions much simpler. If the flutes had to be aligned in a certain pattern, it would most likely require much more attention to detail when making the cushions. If 26

39 more attention were required, then production would slow down greatly. If less time is needed to produce the corrugate cushions, then there is a better chance that corrugate cushions will compete with the more commonly used polymer based cushions (Shin, 2004). Effects of Specimen Size for Corrugate Cushions Research has shown that the size of the test specimen for corrugated may play a role in the results that are collected (Naganathan & Marcondes, 1995). Samples larger than the test platen were tested against samples that matched the size of the drop platen. The oversized samples (larger than the platen) appeared to have much higher acceleration values at lower static loads. It was concluded that the oversized cushion was stiffer than the smaller test sample. The stiffness of the oversized cushion was supported through the generation of dynamic force-deflection curves. The deflection for the oversized cushions was much smaller over a range of force as compared to the standard sized cushions (Naganathan & Marcondes, 1995). For this research, the platen size was 9 inches x 9 inches. The area of the cushions tested never exceeded the size of the drop platen. Enhancement of Cushioning Performance with Paperboard Crumple Inserts It is important that a cushion be able to protect the product against those events that are statistically likely to occur. The likely events are determined through analysis of the distribution data, the fragility of the product, and the size/weight of the product. If it 27

40 can be done at low cost, it would also be ideal to be able to protect against those situations that are less likely to occur but that the product and package may still encounter. For a corrugated cushion, a crumple insert may offer the added protection needed in order to protect the product against these unlikely events without significantly increasing cost (Sek et al, 2005). Corrugated cushions often incur plastic deformation or crushing when heavy impacts occur. Plastic deformation is deformation that is not reversible. Elastic deformation occurs when the cushion returns to its original state after impact. With elastic deformation, the cushion maintains its cushioning abilities and will be able to withstand multiple impacts. A corrugated cushion, as seen in Figure 10, with a crumple element has been designed and tested with the idea of combining both plastic and elastic deformation. (Sek et al, 2005) Figure 10. A cushion created with the combination of a crumple element (virgin corrugate) and a primary element (pre-compressed corrugate) (reprinted with permission from John Wiley and Sons, Inc.) 28

41 The primary element is a set of corrugated layers that has been pre-compressed. This offers a more spring like action as opposed to virgin corrugated samples. There is also a small crumple element that is inserted, which is composed of virgin corrugated that has not been pre-compressed. The idea is that this design will offer protection against both small and large impacts. It was shown that this design could lower the overall shock response spectrum, extend the cushion curve static load range, and provide a significant increase in the allowable drop height for a limited number of extreme cases (Sek et al, 2005). The Effect of Compression of Enclosed Air on the Cushioning Properties of Corrugated Fibreboard It was determined by Marcondes and Naganathan (1995) that cushion curves alone may be inadequate in characterizing the cushioning properties of corrugated cushions if the corrugate cushion is larger in area than the impacting object. Cushion curves were originally designed for closed-cell foams. The size of a corrugated cushion plays a significant role in the cushioning abilities due to the amount of trapped air that is present. Machine direction for corrugate runs with the flutes as seen in Figure

42 Figure 11. Image of the Machine Direction of Corrugated Paperboard If the cushion is larger in the machine direction, then the air has a farther distance to travel. Such a cushion therefore offers better cushioning abilities because as the shock is experienced, the air slowly exits the cushion. The air gradually leaves the cushion as opposed to exiting immediately on impact. The gradual release of air will slow down the product and therefore the intensity of the shock will be lessened. It was also determined that the cushioning properties of a corrugated cushion were improved if the air was further trapped in the cushion by sealing the open ends. This makes the cushion behave similarly to closed-cell foam and therefore improves cushioning ability (Naganathan, He, & Kirkpatrick, 1999). 30

43 CHAPTER THREE MATERIALS AND METHODS Materials The corrugated board was donated to Clemson University from International Paper. The material was shipped in the form of corrugated sheets. The sheets were shipped to Clemson in several corrugated containers to protect the sheets from the distribution environment because it was vital that the flutes avoid being crushed. The sheets were various dimensions and were later cut down to the appropriate size needed to assemble cushions for testing. Basis Weight Determination TIP (TAPPI, 2001) was used to determine the basis weight for each flute type. Five samples of each flute type (A flute, B flute, AC flute, and BC flute) were collected and set to soak in water for 24 hours to separate the medium from the linerboard. Both the linerboard and the fluted medium were patted dry to remove excess moisture. The remaining moisture was removed using a microwave oven. The samples were then placed in the conditioning chamber for at least 24 hours at standard lab conditions (50% RH, 23F). After conditioning, each sample was weighed and the following equation was used to determine the basis weight in lb/msf: Equation 4. sampleweight(g) 453.6(g/lb.) x 144(in. 2 ) 2 x1000 ft samplearea(in. 2 ) 31

44 Construction of Corrugate Cushion Samples Squares were cut from the corrugate sheets using a Kongsberg i-xl 44 cutting table with the multi-cut head for accuracy and speed. Drawings were created using Esko ArtiosCAD software and converted into AI format so that it could be read by the Kongsberg machine The corrugate was cut into squares with the one of following dimensions: 3 x 3, 4 x 4, 5 x 5, and 6 x 6. The dimensions were dictated by the desired static load and dynamic energy level. The samples were then assembled into various thicknesses based on the dynamic energy level that was desired. The squares were glued together using 3M Super multipurpose spray adhesive. The samples were then placed in the standard condition chamber (50% RH, 23F) or the high humidity, high temperature chamber (80% RH, 39F) for at least 72 hours. The samples were tested for cushioning properties within 30 minutes of being removed from the chambers. An example of the cushions used is seen in Figure 12. Figure 12. Image of Cushion Samples before testing 32

45 Cushion Tester The test equipment used was a Lansmont Corporation Cushion Tester, Model 23. The actual cushion tester used is shown in Figure 7. The drop platen on the drop tester is 9 inches x 9 inches. The platen of the cushion tester was instrumented with a PCB ICP piezoelectric accelerometer with Model number 353B15. The shock pulses were captured and analyzed using the GHI WinCAT version software. The software plots the captured pulse as maximum deceleration in the time domain. The shock pulse was filtered to remove noise and the deceleration value was recorded. The equipment used meets the requirements as stated by ASTM D-1596 (ASTM, 2002) for cushion testing. All equipment was calibrated and verified. Initial Test Plan The testing variables included four flute sizes (two single wall, two double wall). The single wall flute sizes were A-flute and B-flute. The double wall flute sizes were AC-Flute and BC-flute. Each flute size was tested at 8-10 energy levels depending on the nature of the material and the results that were collected. It was important to use enough energy levels to fully understand the characteristics of the material. Each flute size would also be tested at two different environmental conditions, with the exception of AC flute, which was only tested at standard conditions. These conditions included standard conditions as stated by ASTM D-4332 (ASTM, 2001) which is 72 o F ± 1 o 50% ± 2% RH. Also, since the overall strength of corrugate was known to deteriorate at 33

46 high moisture levels, the samples were tested at high humidity and high temperature conditions. The high humidity, high temperature chamber was set at 100 o 80% RH which is similar to tropical conditions stated by ASTM D-4332 (ASTM, 2001). Tropical conditions are 104 o F, 90% RH. All samples were subjected to the high temperature, high humidity conditions except for AC flute. The range of energy levels that were tested was created based on previous worked performed by Wenger (1994). The range for corrugate cushions in the flat crush orientation was determined to be between dynamic energy levels of 1 and 25 lbs/in 3. The test plan was then created by evenly separating 8-10 energy levels within this range. The data was then analyzed to determine if additional energy levels were needed to fully characterize the corrugate cushions. The energy levels were created by changing the thickness of the cushion, drop weight, drop height, and/or the surface area to achieve the energy level desired. At each energy level, five cushions were tested for replication purposes. Each cushion was subjected to five drops (if the limit of the accelerometer was not reached before the fifth drop). Corrugate will often give very high acceleration values and the accelerometer has a limited range of acceleration that it can withstand. The accelerometer could withstand up to 500 G s. An acceleration value of 300 G s was considered a reasonable stopping point in order to protect the accelerometer. Once this threshold was reached, the corrugate samples were no longer tested which means that not all samples were subjected to all five drops. 34

47 Testing The drop tests were performed on the Lansmont Corporation Cushion Tester. For each drop, the equivalent free fall drop height (h eq ) and the acceleration value (G s) were recorded. The equivalent free fall drop height was determined by the Lansmont Test Partner Velocity Sensor software version using the impact velocity (v i ) and the force of gravity (g). The program calculated the equivalent free fall drop height using Equation 5. The shock pulse that was captured by the GHI WinCAT version software was filtered based on the duration of the shock pulse. The duration of the shock pulse was determined and the low-pass filter frequency was calculated based on Equation 4 where f f is the filter frequency and the f n is the natural frequency of the shock pulse. Equation 5. f f 10 f n Filtering the shock pulse allowed the elimination of the noise created by the platens and the rods on the test equipment. This allowed a much cleaner shock pulse for determining the acceleration value for the cushion. The equivalent free fall drop height and the acceleration values, along with the static stress and thickness of the cushion were used to determine the dynamic energy (DE) and dynamic stress (DS) for each drop as seen in Equations 6 and 7. Equation 6. DS = Gs 35

48 Equation 7. DE = sh/t The dynamic stress and dynamic energy values were then graphed to determine the equation that would characterize the material for each drop. A trend line that was created for each drop determined the equation. Linear, exponential, and polynomial trend lines were created for each drop. The correlation coefficient, r 2, was also recorded to determine how well the data fit the trend lines. The correlation coefficient is a value from zero to one that tells how well the line represents the data. The closer the r-squared value is to one, the better the trend line represents the data. Microsoft Excel, Version 12 was used for line-fits and to determine the correlation coefficient. From this information, cushion curves were created to help an engineer determine the working range of the material. Since the r square values for the exponential line fit were similar to the polynomial line fit, exponential line fit was used for the statistical analysis. The use of the exponential line fit is explained in more detail later on. Statistical Analysis Linear Regression Regression analysis is a statistical tool that compares a dependent variable with one or more independent variables. Linear regression is a linear model that will determine if two variables are considered statistically different (Mendenhall & Sincich, 1996). 36

49 The plots of the dynamic energy vs. dynamic stress for each flute size at standard conditions were subjected to several line-fit methods including correlation coefficients using Microsoft Excel, Version 12. The line-fit methods included linear, exponential, third order polynomial, and fifth order polynomial trend lines. For each trend line, an r- squared value was also generated. The r-squared values were collected and compared. This was repeated for A-flute, B-flute, and BC-flute at high temperatures and high humidity conditions. Homogeneity of Lines For the homogeneity of lines portion of the statistical analysis, an exponential line fit method was used for comparison. Originally, all of the materials were characterized using a fifth order polynomial line fit based on previous research performed by Wenger (1994). Wenger was able to achieve a correlation coefficient of one with a fourth order polynomial. A correlation coefficient of one is difficult to achieve and typically necessary. A lower correlation coefficient would be acceptable (greater than.85) and may allow the use of a less complex line fit method. After visually analyzing and comparing r- squared values from the regression comparison, it was determined that an exponential line-fit method was comparable to a polynomial line fit. Therefore, for ease of comparison, the exponential line fit was used. Each flute size was compared to one another. Also, standard and high temperature, high humidity for each flute size was compared. 37

50 The exponential curve was linearized by taking the natural logarithm of both sides of Equation 3, which results in Equation 8. A trend line is applied to this linearized data and the slope (b) and y-intercept (ln a) of each trend line were compared. Equation 8. ln y = ln a + bx A SAS output determined if there was a difference between the slopes of the lines or the y-intercepts of the lines. If the p-value was less than 0.05, then it was considered to be significantly different. 38

51 CHAPTER FOUR RESULTS AND DISCUSSION Basis Weight The basis weight of each flute size was determined using TAPPI TIP The results can be seen in Table 1. The relatively large difference between the two linerboards for A-flute is likely due to the bleaching and clay coating of one linerboard. One linerboard had a bright white appearance that was likely intended for printing. The clay coating would add mass to the linerboard and therefore result in a greater basis weight. The fluted medium often contains more recycled content than the linerboard. The increased recycled content often gives the fluted medium a lower basis weight. The B- flute and BC- flute boards that were used also follow this trend. However, the A and ACflute have fluted media that are closer in basis weight to that of the linerboard. It was expected that the linerboards within a flute size would be similar because unbalanced boards can cause the board to warp. (Selke & Twede, 2005) Table 1. Mean Basis Weight Values and standard deviations for A-flute, B-flute, AC-flute, and BC-flute material used for research Flute Size A-flute (lbs./1000 sq ft.) B-flute (lbs./1000 sq ft.) AC-flute (lbs./1000 sq ft.) BC-flute (lbs./1000 sq ft.) Liner (± 0.28) (± 0.51) (± 0.72) (± 0.03) Liner (± 1.57) (± 0.56) (± 0.52) (± 0.01) Liner (± 0.46) (± 0.02) Medium (± 0.03) (± 0.13) (± 0.38) (± 0.05) Medium (± 0.06) (± 0.28) 39

52 The corrugate manufacturer may choose to alter the basis weight for a number of reasons. An increase in basis weight can increase the stacking strength, puncture resistance, and printability of the corrugate. However, the most common basis weight for the linerboard is 42 pounds per thousand square feet. (Selke & Twede, 2005) The most common basis weight for the medium is 26 pounds per thousand square feet. (Selke & Twede, 2005) The basis weight can also play a role in the cushioning properties so the results of this research are specific to the determined basis weights. Summary of Materials Tested For the sake of brevity, A-flute will be used as an example to explain how the cushion curves were created using the stress-energy method. However, the results collected for the other flute sizes tested (B-flute, AC-flute, and BC-flute) can be found in the Appendices. Selection of Dynamic Energy Levels: A-flute The dynamic energy levels selected were based on previous research performed by Wenger (1994). The dynamic energy levels began at a level low enough that the cushions show little to no change in acceleration between drops one through five. When there is little to no change in acceleration, then there is too much cushioning and no deflection is occurring (McKinlay, 2004). In this case, the results are similar to that of having no cushion at all. It is important to establish this lower boundary so the material can be fully characterized. 40

53 The higher end of the energy levels was located where the level is high enough that acceleration exceeded 300 G s and bottoming out occurs after only one or two drops. At this point, there is not enough cushioning for the product and excessive damage will occur because there is too much deflection (McKinlay, 2004). Also, high acceleration values can cause damage to the accelerometer used for testing. Again the accelerometer was limited to 500 G s but for this research 300 G s was considered the stopping point for testing. This was used to establish the upper boundary for the material. For each dynamic energy level, a static stress (weight/area), drop height, and cushion thickness combination was selected to equal the desired energy level as seen in Equation 5. Each of these combinations was repeated five times as stated in ASTM D1596. For A-flute in flat crush mode, the dynamic energy levels ranged from 2 to 25 lbs/in. 3 as seen in Table 2. The energy levels for B, BC, and AC flute are listed in Appendix A. Table 2. Dynamic Energy Levels, including test parameters, tested for A-flute at Standard Conditions Dynamic Energy (inlbs/in 3 ) Drop Height (inches) Weight (lbs.) Area (inches) Static Stress (psi) x x x x x x x x x x Thickness (inches) 41

54 Results from Testing for A-flute at Standard Conditions Results from testing for a dynamic energy of 7.5 are used as an example set and are shown in Table 3. The acceleration values recorded for all dynamic energy levels listed in Table 2 were plotted in Figures 13, 14, and 15. As seen in Table 3, there is a small drop in acceleration from drop one to drop two. This is different from what occurs with some polymer based foam cushions. With polymer based foam cushions, there is typically a gradual increase in acceleration values with subsequent drops. The small decrease in acceleration values for corrugate cushions is likely due to the breakdown of the fibers that make up the corrugate structure. Corrugate is a very strong, rigid structure and energy from the first drop apparently breaks down the structure of the corrugated flutes. The second drop encounters a more spring-like structure and therefore has more deflection. More deflection means that the impact is spread over a longer period of time. If the energy is spread over a greater length of time, then a lower acceleration value is recorded. 42

55 Table 3. An example of data collected for a selected dynamic energy level for A-flute. Expected Dynamic Energy (inlbs/in 3 ) Drop Acceleration Values (G s) Measured Dynamic Energy (inlbs/in 3 ) Dynamic Stress (lbs./in. 2 ) After the second drop, the corrugate flutes are broken down even more. The corrugate starts to lose its resilience and will start to bottom out as early as the third drop. The cushion is offering very little protection between the drop platen and the bottom of the cushion tester because the cushion has lost its resilience. The result of this type of impact is a large acceleration value. Corrugate appears to offer the greatest protection 43

56 during the first three drops. After the third drop, the acceleration values greatly increase. The significant increase in acceleration values means the corrugate cushion did not offer much protection to the product contained in the package. The drop in acceleration values from drop one to drop two occurred in A-flute at energy levels of 7.5 in-lbs/in 3 and below. After an energy level of 7.5 in-lbs/in 3, there was an increase in acceleration values with each subsequent drop. As the energy levels increased, the increase in acceleration values between drops became more dramatic. This increase in acceleration values is to be expected because the increase in weight and drop height is going to do more damage to the corrugate fibers and break down more flutes. If the flutes have lost their resilience, then the cushion is more likely to bottom out. If the cushion bottoms out, the acceleration values will be greater because there is too much deflection. Dynamic Stress vs. Dynamic Energy for A-flute Figures are plots of the data collected from the cushion tester for A-flute corrugate. The dynamic stress values were plotted versus dynamic energy levels. A trendline was applied in order to characterize the material as a cushioning material. Previous research suggested that the nature of the material requires a polynomial line fit in order to represent a significant amount of the data (Wenger, 1994). Therefore, a fifth order polynomial line fit was initially used to characterize the data. However, it was determined that a less complex line fit such as an exponential line fit may be a suitable line fit as well. The use of alternative line fit methods is discussed further in later sections 44

57 but for Figures a fifth order polynomial line fit was applied. The use of the fifth order polynomial made comparison to Wenger s (1994) C-flute results easier as well. Figure 13. Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Drops 1-5 (Polynomial) Drop 1 has been pulled out in Figure 14 to make the shape of the curve more visible. Figure 14 demonstrated why Wenger (1994) might have used a polynomial line fit to characterize the data. However, as discussed later, a fifth-order polynomial may not be needed to describe subsequent drops. 45

58 Figure 14. Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Drop 1 (Polynomial) Figure 15 is the characterization of the average of drops two-five. It is standard practice in industry to take the average of drops two-five. Therefore, the standard practice has been applied here. It appears in Figure 13 that drop two is similar to drops three-five therefore making an average appear to be a good representation. In spite of this apparent similarity, it will be shown later that drop two is in fact different from drops three five. In the appendices, the equations are presented for individual drops. 46

59 Figure 15. Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Avg. of Drops 2-5 (Polynomial) Figure 14 (Dynamic Stress vs. Dynamic Energy for A-flute, Drop 1) has a comparable line fit to the line fit generated by Wenger (1994) for C-flute. The correlation coefficient, r, is slightly higher for Figure 16. However, the correlation for A-flute is still extremely high and considered well comparable. 47

60 Figure 16. Dynamic Stress vs. Dynamic Energy for C-flute at Standard Conditions (Wenger, 1994) Table 4 summarizes the fifth order polynomial equations for drops one, two, three, four, and five as well as the average of drops two-five. All five drops had extremely high correlation coefficients for their trend line meaning the line fit well captured the data collected. 48

61 Table 4. Dynamic Stress vs. Dynamic Energy Equations for A-flute at Standard conditions, Drops 1-5 and Average of Drops 2-5 (Polynomial) Drop Equation R 2 1 y= x x x x x y = x x x x x y = x x x x x y = x x x x x y = x x x x x Avg. 2-5 y = x x x x x The r-squared values have been included as well to show how well the line captured the data. The high r-squared values seen in Table 4 suggested that the polynomial line fit well represents the plotted data. However, as seen later, the exponential line fit does a good job of representing the data with high r-squared values also especially for drops two, three, four, and five. The fifth order polynomial dynamic stress vs. dynamic energy equations and correlation coefficients for B-flute, BC-flute, and AC-flute at standard conditions are in the appendices. Different Line-fit Methods The use of a polynomial line-fit presents some challenges. It is more challenging to use when creating cushion curves. It is also more challenging to compare to polymeric cushions that use an exponential line fit. In order to determine if a different line fit method could be used, the data was subjected to a fifth order polynomial, third order polynomial, exponential, and linear trend lines. The r-squared values for each trend line were collected and compared as seen in Tables Many of the exponential trend lines had good r-squared values and were comparable to the polynomial line-fit. 49

62 However, the polynomial trend line still proved to best describe the data for drop one. It was also observed that the majority of the r-squared values improved from standard conditions to high humidity conditions. One explanation may be the softening of the flutes when subjected to high temperature, high humidity conditions. The high temperature, high humidity cushions do not have to go through the breaking down process as the standard condition cushions do. In other words, the high humidity cushions are not as rigid and have more deflection. However, further research would need to be performed to determine and confirm the cause. Table 5. R-square values for A-flute at Standard Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop Table 6. R-square values for A-flute at High Humidity, High Temperature Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop

63 Table 7. R-square values for B-flute at Standard Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop As seen in Table 7, B-flute demonstrates an extremely low r-squared value for all line-fit methods for drop one. The low r-squared values are due to large variations and lack of a trend in the data as seen in Figure 17. The variation is likely due to the difficulty of reading the shock pulse. With the first drop on B-flute corrugate, there was a lot of high frequency noise and little deflection by the cushion. This made filtering and determination of the peak difficult. The determination of the peak was difficult because there was often more than one peak or a double hump in the shock pulse. The filtering technique used for this research was described in the previous section. It was noted that Wenger (1994) had less scatter in his plots of dynamic stress vs. dynamic energy. Wenger used a fixed filter frequency of 50 Hz. It is unknown if this would reduce scatter or not. 51

64 Figure 17. Dynamic Stress vs. Dynamic Energy for B-flute at Standard conditions Drop one only (Polynomial) Table 8. R-square values for B-flute at High Humidity, High Temperature Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop

65 Table 9. R-square values for BC-flute at Standard Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop Table 10. R-square values for BC-flute at High Humidity, High Temperature Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop Table 11. R-square values for AC-flute at Standard Conditions using exponential, linear, polynomial (3 rd order) and polynomial (5 th order) trend lines Exponential Linear Polynomial (3 rd order) Polynomial (5 th order) Drop Drop Drop Drop Drop With the exception of B-flute, the r-squared values demonstrate that the stress-energy equations for corrugate cushions could possibly be determined using an exponential linefit as opposed to a polynomial line fit. However, it is important to note that B-flute had a poor line fit for drop one for all line fit methods. Further research is needed to understand this difference. A polynomial line-fit was used in this research because previous research by Wenger (1994) used a polynomial line-fit. Figure 18 is a visual representation of the 53

66 trend lines for an exponential line fit for A-flute. Again, with the exception of drop one, the curves appear to well represent the data. The exponential equations for drops one, two, three, four, and five are in Table 12. Figure 18. Dynamic Stress vs. Dynamic Energy for A-flute at Standard Conditions, Drops 1-5 (Exponential) Table 12. Stress-Energy Equations for A-flute at Standard conditions, Drops 1-5 (Exponential) Drop Equation R 2 1 y = 42.76e x y = e x y = e x y = e x y = e 0.267x

67 The r-squared value for each flute size and environmental condition were generated and compiled into Figure 19 for comparison. An r-squared value of less than.85 was highlighted in orange and considered a poor representation of the data. As seen in Figure 19, the fifth order polynomial had acceptable r-squared values for all but one trend line (B-flute, drop one). The third order polynomial had very similar results with the exception of A-flute, drop one which was only slightly under the required.85 r-squared value. The exponential line-fit method also had acceptable r-squared values with the exception of drop one for most flute sizes and drops one-three for b-flute at standard conditions. However, the linear line-fit method was eliminated for further statistical analysis with almost half of the r-square values below.85. Figure 19. The R-squared values for A-flute, B-flute, BC-flute, and AC flute with 5 th order polynomial, 3 rd order polynomial, exponential, and linear line-fit *Bold Lettering represents high temperature, high humidity conditions 55