Unplasticized polyvinyl chloride (UPVC)

The History of PVC

In 1912, a German chemist, Fritz Klatte, discovered the easiest method for PVC industrial production, which is of great significance in the history of PVC industry. However, before him, there are few scientists who contributed to the development of PVC that are worth of mention. In 1835, Henry Victor Regnault, a French scientist explained the existence of the monomer called vinyl chloride. In 1842, Eugen Baumann, a German chemist determined the density and the basic structural formula of PVC resin. He started a detailed research on this material and investigated the result of sunlight exposure of vinyl chloride in 1872 (Americanplasticscouncil, 2005, May). Fritz Klatte’s production method, which is based on the reaction of acetylene and the catalyst at elevated temperature in the presence of hydrogen, was allowed to lapse and the first industrial production commenced in the late 1920s in the USA. In 1931, PVC resin production plant was built in Germany which marked the breakthrough of PVC commercial production. During the 100 years of discovery of PVC resin, this material was just treated as art treasures. The turning point was the appearance of vinyl chloride/vinyl acetate copolymer and the plasticized PVC resin, which provides a processable material at an appropriate melting temperature into a product that is permanently soft and flexible.

In 1928, Union Carbide, an American chemical company, improved the PVC material to make it easily processed for use in lacquers and hard molded articles. In 1932, US Goodrich Corporation discovered that plasticized PVC resin has soft and flexible characteristics and is resistant to corrosion under acid and alkaline solutions. In 1937, The British ICI Company replaced rubber by plasticized PVC substance as layers for wire insulation. At this time, PVC as an effective macromolecular material was put into mass production. The early stage of PVC resin manufacturing process is emulsion polymerization and solution polymerization, and the primary purpose is to replace the production of rubber coating, packaging containers, and other flexible materials. However, the poor performance of emulsion resin and its high level of impurity content and production costs limit its application. In 1941 BF Goodrich produced PVC resin by suspension polymerization which presented better quality than the emulsion polymerization method especially in the aspects of electrical insulation, mechanical strength, and corrosion resistant. Additionally, the suspension polymerization method needs fewer auxiliaries and operates simpler than emulsion polymerization thus was rapidly adopted around the world. This alternative manufacturing method of PVC resin has been further improved especially on the later treatment procedure of polymerization such as drying, packaging, dust reduction and obturated storage and transportation, which greatly promoted PVC resin production in the 1950s. In the 1970s, environmental pollution and waste management became a serious problem, which affected the rapid economic growth of the 1960s. In PVC industry, mercury pollution was the main environmental issue, while the carcinogen of VCM as a severe problem which almost threatened the PVC industry.

Therefore, many countries established regulations to limit VCM content in PVC production areas, and PVC manufacturers invested heavily in improving manufacturing environment. Although the toxicity of VCM reduced the PVC production and increased the manufacturing cost during that period, technological progress of PVC industry has also promoted and these techniques like the PVC suspension polymerization and continuous emulsion polymerization method have been still applied at present. Due to its resistance to light, chemicals, and corrosion, PVC products presented an excellent performance for building applications thus became essential to the construction industry. The low cost, excellent durability and processability of PVC also make it the better option of materials for various industries such as healthcare, IT, transport and textiles. Currently, PVC is the third largest commodity plastic in the world after polyethylene and polypropylene.

PVC creep The creep behavior of PVC material has raised attention since early times. For example in 1950s Faupel has studied PVC pipe creep and stress rupture behaviors under different conditions of static stress and time (Faupel, 1958). Niklas, H. and Eifflaender, K. tested the long-term behavior of PVC and polyethylene pipes loaded with liquid under pressure at various temperatures (Niklas et Eifflaender, 1959). After decades, PVC creep started to be measured at high temperatures close to the glass region (Bergen, 1967). In most engineering applications, Poisson ratio of common structural material is considered as a constant in any deformation analysis. This is the case for most constitutive equations used to describe deformation behavior of solid bodies under loading. However, the dependence of Poisson ratio on time and strain should be considered when dealing with viscoelastic material such as PVC (Bertilsson et al., 1993; Ladizesky et Ward, 1971). The creep phenomena of PVC is much more complex than metal because the volume does not change in most creep phenomena for metal. Several studies show that there is a volume change observed during short-term uniaxial creep tests under tension of PVC and other polymer materials (Bertilsson et al., 1993; Pampillo et Davis, 1971; Pixa, Le Dû et Wippler, 1988).

The time-dependent variation of the volume used in these research studies cannot be described by a simple model because the strain and the deformation ratio are not proportional to each other. Although a relatively large number of experiments were conducted through the years (Mallon, McCammond et Benham, 1972; Shamov, 1965; Theocaris, 1979), appropriate analytical methods for describing these phenomena have yet to be developed. With the development of related research, the temperature has been considered as a variation in both short-term and long-term creep model (Read, Dean et Tomlins, 1994). Besides, the stress relaxation below glass transition temperature has also been investigated under the extension, and the data are obtained from a fixed deformation specimen and fitted with a normal log distribution function (Povolo, Schwartz et Hermida, 1996). As computer technology evolved, finite element Method (FEM) became the tool of choice for the analysis of PVC products (Pantelelis et Kanarachos, 1998; Veronda et Weingarten, 1975). The early creep models used by FE softwares were not appropriate because they were not representative of the real behavior (Sakaguchi et Kaiga, 1986). In addition, most of the cases studied at the time were run under constant conditions of load and temperature leading to limited case analysis. Along with the wide use of polymer products, an increasing number of creep related studies were conducted (Dropik, Johnson et Roth, 2002; Sabuncuoglu, Acar et Silberschmidt, 2011). However, these studies mainly focused on polypropylene and other materials, while limited research was done on PVC. Most of the research work conducted on creep behavior was conducted on the short-term effect limiting the analysis to the primary creep phase, while the long-term behavior was neglected. The creep model calculated by Dropik et al.(2002) is shown as Eq.(0.1), the units of time and stress in this equation are minutes and psi, and the creep data is obtained from tensile test which is shown in Figure 1.2.

Creep model in ANSYS

Creep is described as time-dependent plasticity, at a constant stress and temperature which in ANSYS is included in rate-dependent plasticity. Creep is defined as the deformation of materials, which is dependent on time, stress, and temperature. Therefore, creep is understood as the deformation over time of material under load and temperature but is also a function of the neutron flux level in nuclear applications exposed to radiation (SAS IP, 2016). The von Mises stress is used for creep analysis, and the material is assumed to be isotropic for the von Mises potential. The initial-stiffness Newton Raphson Method is used as the basic solution technique. There are two types of creep rate equations in ANSYS; implicit creep equations and explicit creep equations. The implicit equations are recommended for general application purposes as they are stable, fast, and accurate, especially for long term cases including cases with large creep strain. They can be used to simulate pure creep or creep with isotropic plasticity. The implicit method is more efficient and accurate compared to the explicit method because it can be used to calculate plasticity and creep simultaneously. The temperature-dependent constant is also included in the implicit equations. The explicit method is recommended for cases with small time steps. Unlike the implicit method, there are no temperature-dependent constants, nor is there simultaneous modeling with any other material models. However, the Arrhenius function can be used for temperature dependency and explicit creep can be combined, with other plasticity options, such as transient analysis, by using superposition modeling for the calculations. In these analyses, the program calculates the plastic before the creep. There are several studies that have investigated PVC creep, with most of them focusing on the influence of time or tensile stress. In general, such research implements time hardening creep models or strain hardening creep models (Dropik, Johnson et Roth, 2002; Pulngern et al., 2013).