Elsevier

Composites Part B: Engineering

Volume 37, Issues 2–3, April 2005–March 2006, Pages 249-254
Composites Part B: Engineering

Temperature and time dependence of electrical and mechanical durability of LDPE/diamond composites

https://doi.org/10.1016/j.compositesb.2005.05.014Get rights and content

Abstract

In this study, the dependence of the electrical (τE), mechanical (τσ) life time on electrical strength (E) and mechanical tension (σ) of low density polyethylene composites with commercial diamond additive (LDPE/diamond) at different temperatures are investigated and compared with those of pure LDPE. 50–100 μm thick samples with the additive amount varying between 0.1–3% were prepared by using hot pressing method. log τEf(E, T), E versus diamond additive percentage are drawn and the determined values of mechanical degradation parameters at constant temperature are listed. Since τE and τσ both depend on temperature, electrical degradation mechanism is similar to the mechanical one proving the validity of the thermofluctuation theory. The results indicate that E and σ values reach maximum for 0.5% diamond and are equal to 8.5×107 V/m (6.0×107 V/m for pure LDPE) and 82 MPa (68 MPa for pure LDPE), respectively which complies with our other studies.

Introduction

The combination of conventional polymers with the organic or inorganic additives allows us to create new polymeric materials with unique mechanical, electrical and optical properties. In recent years, despite the extensive usage of synthetic polymers of the class of artificial materials, some problems remain unsolved. Finding the extreme variation mechanism of some polymer characteristics depending on the amount of additive, temperature or manufacturing technology of a polymeric material is especially important. During the use of synthetic pipes in irrigation, they may expose to mechanical stress σ and temperature T. Polymer composites used in the cable industry may be subjected to the electrical strength, σ, T and UV irradiation during electrical discharging.

For polymer dielectrics, the electrical and mechanical durability are characterized with punching and rupture tensions respectively. On the other hand, depending on its using environment, it is very important to investigate durabilities for tensions smaller than electrical punching and mechanical rupture tensions and to comment on τσ, τE from physical point of view.

The σ and E values of materials tested in industry are measured for only one tension. However, mechanical and electrical degradation is not immediate but has a kinetic character. For this reason, in recent years, composite lifetimes are measured at different conditions and more exact results are obtained about degradation mechanism.

In our previous studies [1], [2], [3], [4], [5], [6], [7], the dynamical–mechanical and electrical durabilities and optical properties of polymer composites (based on polyolefin) with different organic (PP/PE fiber) and inorganic (glass fiber) additives are investigated except for LDPE/diamond composites.

The experimental and theoretical investigation of these composites is very important since it makes the degradation mechanism more clear. Commercial diamond, cheap in price, which is already used for the production of synthetic pipes and cables by Dizayn Company1 is investigated for this purpose. All studies including LDPE/diamond composites show that the only factor causing the degradation is the temperature. σ and E are factors helping the degradation and the similarities in their behavior prove that the mechanical and electrical degradation mechanism is same.

Section snippets

Preparation of samples and experimental methods

Composites with different commercial diamond-additive percentages (0.1, 0.3, 0.5, 0.7, 1, 3%) are used in the experiments. The diamond powder is black in color and has a diameter of about 0.1 μm. (with a density of 3.07–3.56×103 kg/m3 and a resistivity 1012–1014 Ωm). The LDPE powder is mixed with diamond mechanically and samples with a thickness of 50–100 μm are prepared with the hot pressing method (15 MPa, 418 K, 10 min). For these samples, a special cell consisting of two stainless steel electrodes

Interpretations of the experimental results

The mechanical and electrical degradation properties were examined for all the composites in our previous studies [1], [2] except LDPE/diamond composites.

For LDPE/diamond composites, the dependence of log τ on σ at constant temperature, is measured and it is seen that the degradation of the composites agrees with Thermofluctuation Theory [10]:τσ=τ0exp[(U0γσ)/RT]

This form of the Thermofluctuation Theory is also valid for electrical punching at certain E and τ intervals. An equation similar to

Conclusion

The mechanical and electrical degradation properties of LDPE vary with additive percentages and have maximum value for 0.5% diamond additive. The parameters χ and γ vary directly with E and σ depending on the amount of the additives. The fact that the initial activation energies (Wo and Uo) for pure and composite samples do not change due to electrical and mechanical degradation, implies that the degradation has a chemical nature. Electrical degradation mechanism is similar to the mechanical

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