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Mechanical and chemical properties of polymer materials are of great importance to know before use in any construction applications. Such materials are increasingly used in infrastructure construction for instance structural components, protective coating, and adhesives.
Mechanical and chemical properties of polymer materials are crucial for designers and engineers since these characteristics demonstrate whether these materials are suitable to be used in construction or not.
High strength or modulus to weight ratios, toughness, resilience, resistance to corrosion, lack of conductivity (heat and electrical), color, transparency are properties which are present in most of polymer materials. It is possible to modify the properties of polymer materials by either adding stabilizer or plasticizers.
Engineers are chiefly concerned with mechanical properties and with the physicochemical which determine durability. The mechanical response of Polymer materials may change appreciably over quite a small temperature range.
Properties of Polymer Materials
The density of polymer materials is low because it mainly composed of light elements. The density of Poly Methyl Pentane is 830 kg per cubic meter, Polypropylene (PP) is 905 kg per cubic meter, and Polytetrafluoroethylene (PTFE) is 2150 kg per cubic meter. These densities are considerably low compared with that of steel which is 7850 kg per cubic meter.
2. Thermal expansion
Thermal expansion of polymers is relatively large. This must be considered in the design and use of polymer components, particularly when used in conjunction with other engineering materials.
Polymers can expand by differing amounts in different directions due to its composition. It contains strong covalent bonds along the polymer chain and much weaker dispersive forces between the polymer chains.
3. Thermal Conductivity
The thermal conductivity (K factor) of polymers is very low. This make it as suitable insulator materials. Polymers also have outstanding electrical insulation properties.
At ambient temperature unfilled polymers have conductivities in the range of 0.15-0.13 W/m°C, about 240 W/m°C and that of copper is about 385 W/m°C W/m°C.
Solid polymers have thermal conductivities in the range of 0.16 to 0.45 W/m/K. In foamed polymers, thermal conductivity is as low as 0.024 W/m/K (watt per metre per degree Kelvin).
Generally, solid polymers do not contain interconnected pores and may generally be regarded as practically impermeable. That is why Polymers are frequently used as protective coatings, vapor barriers, sealants, caulking compounds and proof against gases and vapors.
5. Chemicals Resistant
Polymer can withstand chemicals which makes it a suitable construction material in various circumstances.
There are several types of the strength such as tensile, compression, flexural, torsional, and impact strength. The impact strength is strongly influenced by change in temperature, impact strength generally falling as the temperature falls.
The strength of polymer material is based on molecular weight, cross-linking, and crystallinity. The tensile strength of the polymer rises with increase in molecular weight. Similarly, large molecular weight provides high strength.
Moreover, cross-linking decrease motions of chains and increases the strength of the polymer. The crystallinity of the polymer increases strength, because in the crystalline phase, the intermolecular bonding is more significant.
The durability is based on the type of polymer, its composition and structure, and on the synergistic effect of the conditions of exposure. The durability of a polymer determines whether it is suitable for external construction applications
The changes which bring about environmental degradation in polymers and ultimately determine durability are complex and varied. The complexity arises from the conjoint action of a number of agents of degradation, notably ultraviolet radiation (from sunlight), heat, oxygen, ozone and water.
The list of main agents and modes of degradation in polymer materials is given in table 1.
Table 1: Main agents and modes of degradation in polymer
|Main agents||Mode of degradation|
|Oxygen at moderate temperature||Thermal oxidation|
|Oxygen at higher temperature||Combustion|
|Oxygen + ultraviolet radiation||Photo-oxidation|
|Atmosphere oxygen + water + solar radiation||Weathering atmospheric degradation|
These agents produce physical and chemical changes at the molecular level and these changes differ from one polymer to another. For instance,
Polyethylene is subject to photo-oxidation in sunlight, unless deliberately stabilized by ultraviolet absorbers such as carbon black. As a result, they are more resistant to weathering.
Photo-oxidation and thermal oxidation be reaction of polymer with atmospheric oxygen may bring about cross-linking of polymer chain with accompanying embrittlement or may break chains into small fragments. These fragments are water soluble and re washed out, producing surface erosion.
Ionization radiation also causes molecular damage to polymers, resulting in fusion, cross-linking and degradation of polymer chains. Several polymers fail by slow brittle failure at stresses considerably below the normal failure stress when they are exposed to certain specific organic substances.
This leads to environmental stress cracking (ESC) at points of local stress concentration. Polyethylene (PE) may be made more resistant to stress cracking by increasing the chain length.
Some of the organic monomers from which polymers are synthesized are recognized as toxic and severe controls are placed on the handling of these substances. Residual free monomer levels in thermoplastics are extremely low and these materials are not normally considered hazardous.
However, if exposed to high temperature, partial pyrolytic decomposition may occur, releasing monomer or other volatile and toxic substances. Unpolymerized substances should be handled with strict attention.
Furthermore, toxicity problems arise with certain polymer additives and additives permitted in formulations for contact with potable water should be subject to tight control.