Search Results for polymers

This article provides an overview of polymer fractography, with examples of various fracture surfaces created under diverse loading conditions. The focus is on the interpretation of polymer fracture-surface features in light of the unique viscoelastic nature of polymers. The article presents fractographic examples of three time-dependent cracking mechanisms: fatigue fracture, creep rupture, and environmental stress cracking. It details characteristic fractographic features that can be observed in optical microscopy (OM) and scanning electron microscopy (SEM).

This article discusses the most common thermal analysis methods for thermosetting resins. These include differential scanning calorimetry, thermomechanical analysis, thermogravimetric analysis, and dynamic mechanical analysis. The article also discusses the characterization of uncured thermosetting resins as well as the curing process. Then, the techniques to characterize the physical properties of cured thermosets and composites are presented. Several examples of stress-strain curves are shown for thermosets and thermoplastic polymers.

Rheology is defined as the study of the flow and deformation of matter. This article begins with an examination of flow behavior. It describes the geometries and methods employed for rheological testing of polymers in their molten state. It also discusses materials that are predominantly in the solid state and the methods employed for solids testing. Examples of unidirectional and dynamic oscillatory testing are provided for different mechanical behaviors.

The discussion on the fracture of solid materials, both metals and polymers, customarily begins with a presentation of the stress-strain behavior and of how various conditions such as temperature and strain-rate affect the mechanisms of deformation and fracture. This article describes crazing and fracture in polymeric materials, with a review of the behavior of the elastic modulus as a function of temperature or time parameters, emphasizing the importance of the viscoelastic nature of their deformation and fracture. The discussion covers the behavior of polymers under stress, provides information on ductile and brittle behaviors, and describes craze initiation in polymers and crack formation and fracture by crazing. Macroscopic permanent deformation of polymeric materials caused by shear-yielding and crazing, which eventually can result in fracture and failure, is also covered.

This article reviews generalized test methodologies for fatigue characterization of polymers and examines fatigue fracture mechanisms in different engineering plastics. It provides detailed micromechanistic images of crack-tip processes for a variety of semicrystalline and amorphous engineering polymers. The article describes fracture mechanics solutions and approaches to the fatigue characterization of engineering polymers when dealing with macroscale fatigue crack growth. It includes mechanistic images for high-density polyethylene, ultrahigh-molecular-weight polyethylene, nylon 6, 6, polycarbonate, and polypropylene. The article describes the micromechanisms of toughening of plastics and uses a macroscale approach of applying fracture mechanics to the fatigue life prediction of engineering polymers, building on the mechanistic concepts. It also describes the factors affecting fatigue performance of polymers.

This article describes the processes involved in photochemical aging and weathering of polymeric materials. It explains how solar radiation, especially in the UV range, combines with atmospheric oxygen, driving photooxidation and the development of unstable photoproducts that cause various types of damage when they decompose, including the scission of carbon bonds and polymer chains. The article illustrates some of the degradation reactions that occur in different polymers and presents an overview of the strategies used to prevent such reactions or otherwise mitigate their effects.

Powder bed fusion (PBF) of polymers is a collection of additive manufacturing processes that melt and fuse polymer in a powder bed. This article provides a complete suite of materials and processes involved in PBF of polymers. The discussion includes details of thermal and manufacturing issues, and safety, postprocessing, and finishing considerations, as well as of principal defects in PBF polymer parts and the mechanical properties of the parts produced by PBF. The article provides case studies on the applications of polymer PBF.

Material jetting (MJ) is a classification of additive manufacturing processes that involves the selective jetting and subsequent solidification of liquid droplets onto a substrate in a layerwise manner. This article focuses solely on MJ of polymers, providing a process overview and describing the functional characteristics that distinguish it from other AM technologies. It provides information on the properties and design considerations of both build and support materials. Process-related effects on final part properties and overall quality, as well as corresponding design considerations are also covered. The article also discusses the applications and future scope of polymer MJ systems.

Polymers and polymer composites have become attractive for tribological applications due to their specific material properties. This article begins by discussing the fundamentals of polymer friction and wear. It summarizes the main polymer materials used in tribological applications. The article explains the effects of load, sliding velocity, and temperature on the friction coefficient. It describes three types of wear modes, namely, abrasive, adhesion, and fatigue. The article discusses the frictional behavior of polymer composites and polymer coatings. It concludes by providing information on tribotesting of polymers and polymer composites.

Polymers offer a wide range of choices for medical applications because of their versatility in properties and processing. This article provides an overview of polymeric materials and the characteristics that make them a unique class of materials. It describes the ways to classify polymers, including the polymerization method, how the material deforms, or molecular origin or stability. The article contains tables that list common medical polymers used in medical devices. It explains the medical polymer selection criteria and regulatory aspects of materials selection failure analysis and prevention. Failure analysis and prevention processes to determine the root cause of failures that arise at different stages of the product life cycle are reviewed. The article describes the mechanisms of plastic product failure analysis. It discusses the trends in the use of medical polymers, such as high-performance polymers for implants, tissue engineering, and bioresorbable polymers.

This article discusses several aspects of biocompatibility of polymers, including the selection of a suitable polymer, specific use of a material, contact of polymer on body site, and duration of the contact. It describes the factors influencing the biological response of the polymer from a biocompatibility perspective. These include raw materials, the manufacturing process, cleaning and sterilization processes, and biodegradation and biostability. The article reviews the general testing methods of polymers, such as chemical, mechanical and thermal. It concludes with a section on the guidance, provided by the regulatory authorities, on the biocompatibility testing of polymers and polymer-containing devices that can aid in selecting the right analysis.

Creep deformation is normally studied by applying either a constant load or a constant true stress to a material at a sufficiently high homologous temperature so that a measurable amount of creep strain occurs in a reasonable time. This article provides the phenomenological descriptions of creep and explains the testing and mechanism of creep in crystalline solids. It also presents information on the creep response of crystalline and amorphous solids.

This article reviews the general mechanical properties and test methods commonly used for ceramics and three categories of polymers, namely, fibers, plastics, and elastomers. The mechanical test methods for determining the tensile strength, yield strength, yield point, and elongation of plastics include the short-term tensile test, the compressive strength test, the flexural strength test, and the heat deflection temperature test. The most commonly used tests for impact performance of plastics are the Izod notched-beam test, the Charpy notched-beam test, and the dart penetration test. Two basic test methods for a group or strand of fibers are the single-filament tension and tow tensile tests. Room temperature strength tests, high-temperature strength tests, and proof tests are used for testing the properties of ceramics.

Mechanical properties are often the most important properties in the design and selection of engineering plastics. Temperature, molecular structure, crystallinity, viscoelasticity, and effects of environment, fillers and reinforcements are considered as the basic factors affecting the mechanical properties of engineering plastics. The testing methods for determining mechanical properties, including stress-strain test, modulus-directed tensile test, strength test, strength-directed tensile test, impact test, and dynamic mechanical test are discussed.

In terms of their electrical properties, plastics can be divided into thermosetting and thermoplastic materials, some of which are conductive or semiconductive. This article provides detailed information on factors that affect the property of plastics. It discusses the major test methods used to determine the following dielectric properties of plastics: dielectric breakdown voltage, dielectric strength, dielectric constant, dissipation factor, arc tracking resistance, insulation resistance, volume, and surface resistivity or conductivity. The test specifications and conditions, recommended by several U.S. and foreign testing organizations for characterizing the electrical properties of plastic materials are listed. The article describes the influence of these properties on selection of plastics for insulation application. An outline of the electromagnetic shielding and testing methods of electromagnetic interference are also provided. Designations, electrical properties, and applications of elastomers are tabulated.

Optical testing of plastics includes characterization of materials and analysis of optical components. This article focuses on procedures for testing various characteristics of optical components, including transmission and haze, yellowness, refractive index, birefringence, as well as surface irregularity, contamination, gloss, and color. It provides a short note on ad hoc testing, which is beneficial for practical applications in which lenses, prisms, and light pipes are being used and tested, as other test instruments are often not available.

This article describes weathering and environmental factors that contribute to degradation in plastics, including temperature variations, moisture, sunlight, oxidation, microbiologic attack, and other environmental elements. It presents a general overview of aging factors, their effects on plastic materials, and the accelerated test methods that can be used to estimate the reaction of a plastic component during actual use. The article focuses on the determination of service temperature as it indicates the ability of a material to retain a certain property, when exposed to elevated temperatures for an extended period of time. It concludes by describing various degradation processes, namely, thermal degradation, thermal oxidative degradation, photooxidative degradation, environmental corrosion, and chemical corrosion and discussing the ways of detecting these degradation processes.

Flammability is the ability of a material to undergo easy ignition and rapid flaming combustion. This article provides information on flammability tests of polymers and codes and regulations that cite these tests. Many organizations are involved in the characterization and specification of flammability properties, resulting in several categorization strategies for flammability tests, including tests for specific fire response characteristics, research tests versus acceptance tests, tests for different levels of severity, and tests for basis of origin. The article presents an overview on the basic approaches in improving the fire resistance of polymers and the burning process (heating, decomposition, ignition, combustion, and propagation). It provides a brief description on the test methods which are classified into two types, one based on fire response characteristics and the other on particular applications of polymeric materials.

Tribology is the science and technology of interacting surfaces in relative motion or, the study of friction, wear, and lubrication. This article focuses on friction and wear processes that aid in the evaluation and selection of materials, for polymers and some composites used in friction and wear applications. It provides information on friction, types of wear, and lubrication. The article includes a brief description of the friction and wear test methods, laboratory-scale friction, and wear testing, usually performed either to rank the performance of candidate materials for an application or to investigate a particular wear process. It describes the wear tests conducted with/without abrasives and explains the concept of PV limit (where P is contact pressure and V is velocity). The article concludes with references and tables of friction and wear test data for polymeric materials.

Thermal analysis provides a powerful tool for researchers and engineers in determining both unknown and reproducible behavioral properties of polymer molecules. This article covers the thermal analysis and thermal properties of engineering plastics with respect to chemical composition, chain configuration, conformation of the base polymers, processing of the base polymers with or without additives; and the response to chemical, physical, and mechanical stresses of base polymers as unfilled, shaped articles or as components of composite structures. It also describes thermal analysis techniques, including differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and rheological analysis. This article also summarizes the basic thermal properties used in the application of engineering plastics, such as thermal conductivity, temperature resistance, thermal expansion, specific heat, and the determination of glass transition temperatures. It concludes with a discussion of the thermal and related properties of nine thermostat resin systems divided into three groups by low, medium, and high service temperature capabilities.