Engineering plastics thermal expansion

One particular feature of PPO is its exceptional dimensional stability amongst the so-called engineering plastics. It has a low coefficient of thermal expansion, low moulding shrinkage and low water absorption, thus enabling moulding to close tolerances. 

It is quite common in modem engineering designs, for plastics to be used in conjunction with other materials, particularly metals. In such cases it is wise to consider the possibility of thermal stresses being set up due to the differences in the thermal expansion (or contraction) in each material. 

Simha, R., and Jain, R. K., Statistical approach for polymer melt composites bulk modulus and thermal expansivity, in Society of Plastics Engineers, National Technical Conference Proceedings, (1982), pp. 78-81. 

Polymeric materials have relatively large thermal expansion. However, by incorporating fillers of low a in typical plastics, it is possible to produce a composite having a value of a only one-fifth of the unfilled plastics. Recently the thermal expansivity of a number of in situ composites of polymer liquid crystals and engineering plastics has been studied [14,16, 98, 99]. Choy et al [99] have attempted to correlate the thermal expansivity of a blend with those of its constituents using the Schapery equation for continuous fiber reinforced composites [100] as the PLC fibrils in blends studied are essentially continuous at the draw ratio of 2 = 15. Other authors [14,99] observed that the Takayanagi model [101] explains the thermal expansion. 

The model Is based on elastic-plastic fracture mechanics principles, and Incorporates effects associated with thermal expansion mismatch and modulus mismatch of various constituents, as well as non-linear material behavior as a function of load and temperature. Key properties of the constituents, such as those of the interlayer, reaction zone, and base material are provided as a data base these data were measured in this program by using bulk samples, The model then uses the processing history, specimen geometry and loading conditions to evaluate the performance of the joint, The results of finite element analysis of cracked specimens have been consolidated In arriving at the engineering model, JADM,... 

Epoxy resin, as an adhesive, is nsed for many applications in building/construction, (i.e., it is possible to bond a new rebar to existing steel in concrete instead of welding it, by use of special epoxy adhesives). Epoxy can bond to almost any material (for strnctural or non-structural bonding) with high adhesive strength in various environments and temperatures. In civil engineering applications, epoxy adhesives are used to bond concrete in a number of different ways. Epoxy can be used to bond plastic concrete (or wet concrete) to cured concrete, it can be used to bond cured concrete to cured concrete, or cured concrete to cracked concrete, as well as to bond cured concrete to other materials with similar or dissimilar thermal expansion coefficients and elastic moduli. 

TABLE 15.9 Thermal Expansion and Water Absorption of Glass-Fiber-Reinforced Engineering Plastics... 

Fillers usually have a thermal conductivity about 20 times higher than plastics, and the specific heat is about 50%. By improving the heat transfer in the melt, the use of a filler may therefore give a faster set-up when moulding, and so improve the cycle time. In applications, the same effect may be useful in engineering components, improving heat dissipation and/or producing a thermal expansion closer to that of metal. 

Induced stress is always a factor in structural applications and can result from processing conditions, thermal history, phase transitions, surface degradation, and variations in the expansion coefficient of components in a composite. Since the modulus of a material is not only temperature dependent but time dependent as well, the stress relaxation behaviour of polymers and composites is of great importance to the structural engineer. Stress relaxation is also important to the polymer chemist developing new engineering plastics because relaxation times and moduli are affected by polymer structures and transition temperatures. Therefore, it is essential that polymeric materials that will be subjected to loading stress be characterised for stress relaxation and creep behaviour. 

Figure 7-3, Nusselt-Graetz relationship for molten or thermally softened flowing polymers with viscous dissipation and thermal expansion effects. (Reproduced with permission from references 6 and 7. Copyrights 1972 and 1975, Society of Plastics Engineers.)...

Compared to metals and many other engineering materials, plastics have lower tensile strength, lower elastic modulus, and a higher coefficient of thermal expansion. These differences strongly influence the way joints are designed and adhesives are selected. In the paragraphs that follow the significance of these differences will be briefly analyzed. 

The coefficient of thermal expansion is defined as the fractional change in length or volume of a material for a unit change in temperature. The coefficient of thermal expansion values for different plastics are of considerable interest to design engineers. Plastics tend to expand and contract anywhere from six to nine times more than materials such as metals. This difference in the coefficient of expansion... 

When materials with different coefficients of thermal expansion (CTE) are joined, shear stresses result when the assembly is heated or cooled. Many engineering plastics have a CTE value in the range 80-100 x 10 mm/mm/°C but sometimes differences can occur. Eor example, liquid crystal polymer has a CTE of 10 x 10 mm/mm/°C, whereas acrylic has a CTE of 80 x 10 mm/mm/°C, and if these two substrates were to be bonded with a cyanoacrylate (CTE = 80 x 10 mm/mm/°C [5]) then the adhesive could be subjected to some quite severe stresses at the extreme operating temperature range. In this case a thicker bond line and more compliant or flexible adhesive (e.g., a flexible UV acrylic) may reduce problems. 

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