Chemical Education Today
Report
Impact of Polymers in Impact Sports by Sandy Van Natta and John P. Williams
Sports! Think of fun, exercise, a feeling of health and well being; a wonderful pastime whether as participant or observer. National Chemistry Week picks up on the popularity of sports with its 2008 theme, “Having a Ball with Chemistry”. However, despite their popularity, in many sports the participants are susceptible to various impacts or sudden blows— from being hit by projectiles such as balls, from bumping or crashing into objects, or from colliding with other participants. In order to minimize sports-related injuries, a variety of protective body equipment has become an integral (and often required) component. Since much of this modern body armor is made from polymers, we will discuss some representative types and forms of polymers that are used (1). And since the head is the most vulnerable part of the body, we will focus on the use of polymers to design protective headgear—helmets. Polymer Basics Synthetic polymers (2) play an integral role in many aspects of our everyday lives. Polymers are very large molecules (macromolecules) that are made from hundreds to hundreds of thousands of smaller repeating units known as monomers. For example, polyethylene is made up of long chains of the monomer ethylene. (Note that the polymer name is based on the monomer name.) Common names and abbreviations of polymers referred to in this article include polyethylene (PE); polypropylene (PP); polycarbonate (Lexan); poly(methyl methacrylate) (Plexiglas); acrylonitrile butadiene styrene (ABS); polystyrene (PS); polyurethane (PU); and polyvinyl chloride (PVC). Polymers exhibit a wide variety of properties. For example, the polymers listed in the previous paragraph are all moldable. This property allows them to soften upon heating, undergo shaping, then resolidify in the new shape upon cooling. Properties partially depend on the monomer from which the polymer was made. For example, polycarbonate (Lexan) is a rigid plastic that is strong and durable, yet clear and lightweight, properties that make it useful in applications such as the lenses of motorcycle goggles. Polyethylene is a flexible plastic that can be deformed rather easily, yet resists breakage; this makes it a good choice for product packaging such as milk jugs. Polymer properties can also be adjusted by combining the polymer with other substances. For example, PVC is a rigid plastic suitable for water pipes. However, adding a substance called a plasticizer to PVC produces a plastic that is flexible enough to use for inflatable toys. The properties of polymers also depend on the process(es) by which the polymer has been formed. For example, polyethylene can be formed by a process called blowmolding in which high-pressure air is used to expand the softened plastic against the inside of a mold. This process allows more polymer chains to be oriented parallel with one another, which increases the strength of the resulting polymer. When gas bubbles are present during the polymerization process, polymers can be produced as a foam. The gas may be a 1326
Figure 1. Polymer foams may have open (flexible) or closed (rigid) cells, or a combination of both. An open-cell polyurethane is shown on the left, a closed-cell polyurethane on the right. Graphic provided by L. J. Gibson and M. F. Ashby; reprinted with permission of Cambridge University Press (3).
result of the polymerization process itself or from the addition of a liquid (called a blowing agent) that vaporizes during polymerization. The physical properties of a foam of a particular polymer can vary widely, depending on the specific reaction conditions as well as the nature of other additives. If most of the cells or cavities formed by the gas bubbles are open (i.e. have popped), the result is a flexible foam, such as those used in cushions and certain packaging materials. If most of the cells or cavities remain intact and closed, the result is a rigid foam, such as those used for egg cartons, ice coolers, and certain hot drink cups. Figure 1 shows both open-cell and closed-cell polyurethane foams. Providing Needed Protection In the U.S., brain injuries are the leading killer of and cause of disability in children and young adults. Up to 20% of these injuries occur in sports (in which more than 30 million children participate) and recreation (4b). On-the-field concussions in football, of which approximately 100,000 occur in the U.S. each year, are considered one of the most serious of contact sports injuries (5). According to recent research, concussions are most likely to happen as the result of a blow to the side of the head, rather than the front or top. When an unprotected head strikes against a hard surface, the brain slams against the skull as a result of inertia; this can result in brain injury. A helmet typically has an inner lining and outer shell. Each part serves a different safety function. The function of the outer shell is to spread the energy from an impact over a larger region and to provide resistance to penetration. The shell also serves as a base to which the inner lining and other helmet parts may be attached. The important function of the helmet inner lining is to bring the head (and brain) to a more gradual stop upon impact. The foam pads found on the inside of most helmets attenuate the impact by being compressed gradually, thus extending the stopping process from about 1 ms to 6 ms. Reducing the spike of energy to the head and brain decreases the chance of permanent brain damage. A helmet also performs “energy management” by converting some of the energy of the collision to heat.
Journal of Chemical Education • Vol. 85 No. 10 October 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
Chemical Education Today
Polymers Used in Helmets Helmets used in different sports vary by using different polymers for the outer shell and inner lining, depending on the impacts encountered in the particular sport. For example, football helmets commonly have a polycarbonate outer shell, and an inner liner of PU, PP, or PE. Bicycle helmets commonly have outer shells comprised of ABS or polycarbonate, and an inner liner of PS, PU, or PE. For the shells, polycarbonate and ABS are lightweight and impact resistant. In a football helmet, flexible PU, PP, and PE foams absorb impact but still rebound. Rigid PU, PS, and PE foams in bicycle helmets absorb impact with little rebound. Figure 2 shows photographs of the inside and outside of these two types of helmets as well as a molecular view of a rigid polymer (polystyrene) and a flexible polymer (polypropylene). The two categories of foam used in helmet design are flexible and spongy, and rigid but crushable. Flexible and spongy foams readily compress in milder impacts, giving better protection against lesser injuries. Since this type of foam returns to its original shape (recovers), it can be used for protection against repeated impacts, as in football helmets. Rigid foams are best for helmets designed to protect against a single, very hard impact, as in bike helmets. When the impact is hard enough to crush rigid foam, the foam does not bounce back like a spring, which would worsen the impact to the head and brain. Such a foam needs to be replaced after every severe impact. With the many different types of foams available, a helmet lining can be designed for any particular range of impacts between these two extremes (6). Materials Testing The outer shell of a helmet is designed to provide impact resistance. Materials for the shell are evaluated based on their
response to the energy of a shock or sudden stress during impact tests. To select material to be used in a commercial product such as a helmet, most product engineers rely not only on results from impact tests, but also on stress/strain, tensile strength, ductility, and thermal sensitivity data (7). The Gardener Impact Test (8) can be used to evaluate the toughness of polymer samples. Toughness is a measure of the energy a sample can absorb before it breaks (9), which depends upon both its strength (how much force per cross-sectional area it can withstand) and its resistance to deformation. In the test, a weight is dropped vertically from measured heights onto a sample, with a tube or rails to guide the weight. After each drop, the sample is examined for failure; this could be an indentation evident on the back side of the sample, the beginnings of a crack, or complete breakage. Industry tests are typically conducted at 68 °F (20 °C) using a 3 × 5-inch sample with a thickness of 1/16 to 1/4-inch. The unit of measure of impact is the inch–pound (a 10-pound weight dropped from a height of an inch onto the sample equals 10 in.–lb.). Testing Standards for Helmets Materials to be used in helmets are tested for their suitability, and then the finished products are subjected to rigorous tests. A number of organizations (4) have established testing procedures and standards. For example, both the National Collegiate Athletic Association and the National Federation of State High School Associations (10b) have adopted standards developed by the National Operating Committee on Standards for Athletic Equipment (NOCSAE) (10a–c) for various helmets (and face masks). For example, NOCSAE test standards (10c) for football helmets involve dropping a helmet that is protecting the head of a model onto a firm rubber pad. The model head and
polycarbonate (Lexan)
polypropylene (PP)
polystyrene (PS)
photos J. W. Moore & R. J. Wildman
Figure 2. Outer shells of helmets are often made of rigid materials such as polycarbonate (top). Inner liners of football helmets have flexible foams of polymers such as polypropylene (center). Liners of bicycling helmets are made of more rigid foam designed to absorb impact with little rebound, such as polystyrene (bottom).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 10 October 2008 • Journal of Chemical Education
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Chemical Education Today
Report helmet are dropped 16 times from a height of 60 inches1 (1.5 m) onto 6 locations on the helmet. An illustration of a model head and helmet in the six locations is available (11). Most of the drops are conducted at ambient temperatures, with at least two drops conducted immediately after the helmet has been at a high temperature (120 °F; 49 °C) for four hours. Drops from this height are equivalent to a player running at 18 ft/s (12 mph; 5.5 m/s) into a surface that stopped the player’s head in less than one inch (2.54 cm). Although most players run faster than this (average speed of running 40 yards in 4.8 seconds is 25 ft/sec), it is very rare the head is stopped in such a short distance on the football field (10a). Students Become Product Engineers Two inquiry-based activities2 included as an online supplement allow students to experience testing usually performed by product engineers. Students investigate various polymers used in sports helmets. They perform an impact toughness test for “hard” plastics that might be used for a helmet’s outer shell, and an impact attenuation test for foamed plastics that might be used as the inner lining. The tests, in which a plumb bob is dropped onto various samples, is modeled after the actual industrial testing methods described earlier. The impact attenuation test adds a small chocolate bar underneath the foamed plastics. After the plumb bob is dropped onto the foam, students see whether the impact through the foam broke the bar. Students use their data to identify which plastics are most suitable for the two parts of a helmet. Figure 3 shows the apparatus for the impact toughness test. The apparatus can be constructed using materials available in hardware stores. Polymer samples can often be collected locally and are also available commercially. Activity kits are also available from a science supplier. Information on the materials and kits is available in the online supplement.
parts needed
A discussion of the use of science and technology to aid and protect humans can flow from these activities. As students examine and determine the properties of various materials, they mimic the scientific and technological processes involved in designing a consumer product for market. Notes 1. Units are given in the U.S. (British) system in preference to metric or SI because Impact Standard testing is done in the U.S. system. 2. The activities are representative of those developed in the Polymer Ambassador program (http://www.polymerambassadors.org; accessed May 2008). Formed in 1991, the program’s mission is to promote polymer education with teachers, students, and community audiences using resources from educational, industrial, and professional societies.
Literature Cited 1. Van Natta, S.; Ryan, T. Modern Body Armor. Presented at the 225th ACS National Meeting, New Orleans, LA, March 23–27, 2003. A PowerPoint with information from the presentation is available at http://polymerambassadors.org/ModernBodyArmorall. pdf (accessed Aug 2008). 2. Several references that provide extensive information about polymers in general as well as other polymer activities: (a) Chain Gang—The Chemistry of Polymers, ISBN 1-883822-13-0, Terrific Science Press, 1995, http://www.terrificscience.com/sciencestore/ product.php?pid=28 (accessed Jul 2008). (b) Hands on Plastics Science Education Web site, American Chemistry Council, 1300 Wilson Blvd. Arlington, VA 22209, 2007. http://www.americanchemistry.com/s_plastics/sec_content.asp?CID=1123&DID=4277 (accessed Jul 2008). (c) Comprehensive list of polymer references in the education literature, Hodgson, S. C.; Bigger, S. W.; Billingham, N. C. J. Chem. Educ. 2001, 78, 555. (d) Kundell, F. A. Polymer Primer. This is a complete text giving an overview of
apparatus set-up
30r long PVC tube approximately 11/4r diameter marked in 6” intervals
plumb bob connected to 36r piece of string 30r
8 oz (227 g) plumb bob on a string about 36r long
plastic sample to be tested
24r 18r
not to scale
12r
sample support (e.g. jar lid)
wood block 3r t 3r t 1r
3r
6r
plastic sample to test sample support wood block
3r
Figure 3. Apparatus students use for an impact toughness test.
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Chemical Education Today
3. 4.
5. 6. 7.
8. 9. 10.
polymer chemistry and engineering, available at http://www.jce. divched.org/JCEBooks/polymers.html. Gibson, L. J.; Ashby, M. F. Cellular Solids, 2nd ed.; Cambridge University Press: New York, 1997; p 21. American Chemistry Council, 1300 Wilson Blvd, Arlington, VA 22209, 2007. (a) Gear Up for Safety: Helmets. http://www. americanchemistry.com/s_plastics/interactive_media/GearUpForSafety_Plain/helmets.asp (accessed Jul 2008). (b) Gear Up for Safety: Know the Score. http://www.americanchemistry.com/s_ plastics/interactive_media/GearUpForSafety_Plain/knowthescore. asp (accessed Jul 2008). The New Revolution Helmet. http://riddell.com/revfacts.htm (accessed Jul 2008). Foams Used in Bicycle Helmets. http://www.bhsi.org/foam.htm (accessed Jul 2008). (a) “Polymer Research and Development,” a unit of Science in a Technical World, a Project of the Education Division of the American Chemical Society, Teacher’s Edition ISBN: 0-71673550-4, W. H. Freeman and Company, http://www.whfreeman. com/stw/avail.htm (accessed Jul 2008). (b) ASTM International— Standards Worldwide. http://www.astm.org (accessed Jul 2008). Uninstrumented Falling Weight, Gardener or Gardner Testing. http://www.instron.com/wa/products/impact/gardener.aspx (accessed Jul 2008). Macrogalleria Level 3: How They Work. http://www.pslc.ws/ macrog/level3.htm (accessed Jul 2008). (a) National Operating Committee on Standards for Athletic Equipment. http://www.nocsae.org/index.html (accessed Jul 2008). (b) Frequently Asked Questions and Answers. http://www.nocsae. org/faq/index.html (accessed Jul 2008). (c) Standard Drop Test
Method and Equipment Used in Evaluating the Performance Characteristics of Protective Head Gear, NOCSAE DOC (ND) 001-06m07, June 2007; Standard Performance Specification for Newly Manufactured Football Helmets, NOCSAE DOC (ND) 002-98m05, July 2005; http://www.nocsae.org/standards/documents.html (accessed May 2008). 11. Standard Performance Specification for Newly Manufactured Football Helmets (fig. 1). http://www.nocsae.org/standards/pdfs/ Standards%20’06/ND002-98m05.pdf (accessed Jul 2008).
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Oct/abs1326.html Abstract and keywords Full text (PDF) with links to cited URLs and JCE articles Supplements
Two activities where students perform impact tests on possible materials for use in the outer shells and inner linings of helmets. Includes instructor notes and data tables for students.
Fully manipulable ( Jmol) versions of many polymers are part of the JCE Featured Molecules collection on JCE Online at http:// www.JCE.DivCHED.org/JCEWWW/Features/MonthlyMolecules/. This month’s column on p 1456 expands that collection.
Sandy Van Natta is in the Department of Teacher Education, Miami University Hamilton, Hamilton, OH 45011;
[email protected]. John Williams is in the Department of Chemistry and Biochemistry, Miami University Hamilton, Hamilton, OH 45011.
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