1.2 Polymer Categories
1.2.1 Acetal (POM)
Acetal polymers are formed from the polymerization of formaldehyde. They are also known by the name polyoxymethylenes (POM). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s when DuPont developed Delrin.1 Homopolymers are prepared from very pure formaldehyde by anionic polymerization, as shown in Fig.
1.4. Amines and the soluble salts of alkali metals catalyze the reaction.2
The polymer formed is insoluble and is removed as the reaction pro- ceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldhyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhy- dride. An alternative method to improve the thermal stability is copoly-
Figure 1.3 Isotactic, syndiotactic, and atactic polymer chains.
merization with a second monomer such as ethylene oxide. The copoly- mer is prepared by cationic methods.3 This was developed by Celanese and marketed under the tradename Celcon. Hostaform is another copolymer marketed by Hoescht. The presence of the second monomer reduces the tendency for the polymer to degrade by unzipping.4
There are four processes for the thermal degradation of acetal resins. The first is thermal or base-catalyzed depolymerization from the chain, resulting in the release of formaldehyde. End capping the polymer chain will reduce this tendency. The second is oxidative attack at random positions, again leading to depolymerization. The use of antioxidants will reduce this degradation mechanism. Copolymerization is also helpful. The third mechanism is cleavage of the acetal linkage by acids. It is, therefore, important not to process acetals in equipment used for polyvinyl chloride (PVC), unless it has been cleaned, due to the possible presence of traces of HCl. The fourth degradation mechanism is thermal depolymerization at temperatures
Figure 1.4 Polymerization of formaldehyde to polyoxymethylene.
above 270°C. It is important that processing temperatures remain below this temperature to avoid degradation of the polymer.5
Acetals are highly crystalline, typically 75% crystalline, with a melt- ing point of 180°C.6 Compared to polyethylene (PE), the chains pack closer together because of the shorter C—O bond. As a result, the poly- mer has a higher melting point. It is also harder than PE. The high degree of crystallinity imparts good solvent resistance to acetal poly- mers. The polymer is essentially linear with molecular weights (Mn) in the range of 20,000 to 110,000.7
Acetal resins are strong and stiff thermoplastics with good fatigue properties and dimensional stability. They also have a low coefficient of friction and good heat resistance.8 Acetal resins are considered sim- ilar to nylons, but are better in fatigue, creep, stiffness, and water resistance.9 Acetal resins do not, however, have the creep resistance of polycarbonate. As mentioned previously, acetal resins have excellent solvent resistance with no organic solvents found below 70°C, howev- er, swelling may occur in some solvents. Acetal resins are susceptible to strong acids and alkalis, as well as oxidizing agents. Although the C—O bond is polar, it is balanced and much less polar than the car- bonyl group present in nylon. As a result, acetal resins have relatively low water absorption. The small amount of moisture absorbed may cause swelling and dimensional changes, but will not degrade the poly- mer by hydrolysis.10 The effects of moisture are considerably less dra- matic than for nylon polymers. Ultraviolet light may cause degradation, which can be reduced by the addition of carbon black. The copolymers generally have similar properties, but the homopolymer may have slightly better mechanical properties, and higher melting point, but poorer thermal stability and poorer alkali resistance.11
Along with both homopolymers and copolymers, there are also filled materials (glass, fluoropolymer, aramid fiber, and other fillers), tough- ened grades, and ultraviolet (UV) stabilized grades.12 Blends of acetal with polyurethane elastomers show improved toughness and are avail- able commercially.
Acetal resins are available for injection molding, blow molding, and extrusion. During processing it is important to avoid overheating or the production of formaldehyde may cause serious pressure buildup. The polymer should be purged from the machine before shutdown to avoid excessive heating during startup.13 Acetal resins should be stored in a
dry place. The apparent viscosity of acetal resins is less dependent on shear stress and temperature than polyolefins, but the melt has low elasticity and melt strength. The low melt strength is a problem for blow molding applications. For blow molding applications, copolymers with branched structures are available. Crystallization occurs rapidly with postmold shrinkage complete within 48 h of molding. Because of the rapid crystallization it is difficult to obtain clear films.14
The market demand for acetal resins in the United States and Canada was 368 million pounds in 1997.15 Applications for acetal resins include gears, rollers, plumbing components, pump parts, fan blades, blow-molded aerosol containers, and molded sprockets and chains. They are often used as direct replacements for metal. Most of the acetal resins are processed by injection molding, with the remain- der used in extruded sheet and rod. Their low coefficient of friction make acetal resins good for bearings.16
1.2.2 Biodegradable polymers
Disposal of solid waste is a challenging problem. The United States consumes over 53 billion pounds of polymers a year for a variety of applications.17 When the life cycle of these polymeric parts is complet- ed they may end up in a landfill. Plastics are often selected for appli- cations based on their stability to degradation, however, this means degradation will be very slow, adding to the solid waste problem. Methods to reduce the amount of solid waste include either recycling or biodegradation.18 Considerable work has been done to recycle plas- tics, both in the manufacturing and consumer area. Biodegradable materials offer another way to reduce the solid waste problem. Most waste is disposed of by burial in a landfill. Under these conditions oxy- gen is depleted and biodegradation must proceed without the presence of oxygen.19 An alternative is aerobic composting. In selecting a poly- mer that will undergo biodegradation it is important to ascertain the method of disposal. Will the polymer be degraded in the presence of oxygen and water, and what will be the pH level? Biodegradation can be separated into two types—chemical and microbial degradation. Chemical degradation includes degradation by oxidation, photodegra- dation, thermal degradation, and hydrolysis. Microbial degradation can include both fungi and bacteria. The susceptibility of a polymer to biodegradation depends on the structure of the backbone.20 For exam- ple, polymers with hydrolyzable backbones can be attacked by acids or bases, breaking down the molecular weight. They are, therefore, more likely to be degraded. Polymers that fit into this category include most natural-based polymers, such as polysaccharides, and synthetic mate- rials, such as polyurethanes, polyamides, polyesters, and polyethers.
Polymers that contain only carbon groups in the backbone are more resistant to biodegradation.
Photodegradation can be accomplished by using polymers that are
unstable to light sources or by the use of additives that undergo photo- degradation. Copolymers of divinyl ketone with styrene, ethylene, or polypropylene (Eco Atlantic) are examples of materials that are sus- ceptible to photodegradation.21 The addition of a UV-absorbing mate- rial will also act to enhance photodegradation of a polymer. An example is the addition of iron dithiocarbamate.22 The degradation must be controlled to ensure that the polymer does not degrade pre- maturely.
Many polymers described elsewhere in this book can be considered for biodegradable applications. Polyvinyl alcohol has been considered in applications requiring biodegradation because of its water solubil- ity. However, the actual degradation of the polymer chain may be slow.23 Polyvinyl alcohol is a semicrystalline polymer synthesized from polyvinyl acetate. The properties are governed by the molecular weight and by the amount of hydrolysis. Water soluble polyvinyl alco- hol has a degree of hydrolysis 87 to 89%. Water insoluble polymers are formed if the degree of hydrolysis is greater than 89%.24
Cellulose-based polymers are some of the more widely available, nat- urally based polymers. They can, therefore, be used in applications requiring biodegradation. For example, regenerated cellulose is used in packaging applications.25 A biodegradable grade of cellulose acetate is available from Rhone-Poulenc (Bioceta and Biocellat), where an addi- tive acts to enhance the biodegradation.26 This material finds applica- tion in blister packaging, transparent window envelopes, and other packaging applications.
Starch-based products are also available for applications requiring biodegradability. The starch is often blended with polymers for better properties. For example, polyethylene films containing between 5 to
10% cornstarch have been used in biodegradable applications. Blends of starch with vinyl alcohol are produced by Fertec (Italy) and used in both film and solid product applications.27 The content of starch in these blends can range up to 50% by weight and the materials can be processed on conventional processing equipment. A product developed by Warner-Lambert, called Novon, is also a blend of polymer and starch, but the starch contents in Novon are higher than in the mate- rial by Fertec. In some cases the content can be over 80% starch.28
Polylactides (PLA) and copolymers are also of interest in biodegrad- able applications. This material is a thermoplastic polyester synthe- sized from the ring opening of lactides. Lactides are cyclic diesters of lactic acid.29 A similar material to polylactide is polyglycolide (PGA).
PGA is also a thermoplastic polyester, but one that is formed from gly- colic acids. Both PLA and PGA are highly crystalline materials. These materials find application in surgical sutures, resorbable plates and screws for fractures, and new applications in food packaging are also being investigated.
Polycaprolactones are also considered in biodegradable applications such as films and slow-release matrices for pharmaceuticals and fer- tilizers.30 Polycaprolactone is produced through ring opening polymer- ization of lactone rings with a typical molecular weight in the range of
15,000 to 40,000.31 It is a linear, semicrystalline polymer with a melt- ing point near 62°C and a glass transition temperature about —60°C.32
A more recent biodegradable polymer is polyhydroxybutyrate- valerate copolymer (PHBV). These copolymers differ from many of the typical plastic materials in that they are produced through bio- chemical means. It is produced commercially by ICI using the bacte- ria Alcaligenes eutrophus, which is fed a carbohydrate. The bacteria produce polyesters, which are harvested at the end of the process.33
When the bacteria are fed glucose, the pure polyhydroxybutyrate polymer is formed, while a mixed feed of glucose and propionic acid will produce the copolymers.34 Different grades are commercially available that vary in the amount of hydroxyvalerate units and the presence of plasticizers. The pure hydroxybutyrate polymer has a melting point between 173 and 180° C and a Tg near 5° C.35
Copolymers with hydroxyvalerate have reduced melting points,
greater flexibility and impact strength, but lower modulus and ten- sile strength. The level of hydroxyvalerate is 5 to 12%. These copoly- mers are fully degradable in many microbial environments. Processing of PHBV copolymers requires careful control of the process temperatures. The material will degrade above 195°C, so processing temperatures should be kept below 180°C and the pro- cessing time kept to a minimum. It is more difficult to process unplasticized copolymers with lower hydroxyvalerate content because of the higher processing temperatures required. Applications for PHBV copolymers include shampoo bottles, cosmetic packaging, and as a laminating coating for paper products.36
Other biodegradable polymers include Konjac, a water-soluble nat- ural polysaccharide produced by FMC, Chitin, another polysaccharide that is insoluble in water, and Chitosan, which is soluble in water.37
Chitin is found in insect exoskeletons and in shellfish. Chitosan can be formed from chitin and is also found in fungal cell walls.38 Chitin is used in many biomedical applications, including dialysis membranes, bacteriostatic agents, and wound dressings. Other applications include cosmetics, water treatment, adhesives, and fungicides.