Thermal Properties and others
Polypropylene has a high melting point and, thus, it keeps and excellent mechanical resilience even at high temperatures. Due to the distribution of crystals sizes, polymer materials do not show a sole melting temperature but a range of temperatures. Generally, the temperature at which the last crystal is melted is taken as the melting temperature. The range of melting temperature of a homopolymer goes from 165°C to 175°C.
Heat distortion temperature (HDT) is an important property when assessing the materials sensibility to thermal variations. That distortion temperature is the one at which a specimen of 5 x 1/2 x 1/4 inches deflects under a placed load of 66 or 264 psi (ASTM D648). This measure is only useful for quality assurance of different batches of products and it should not be taken into account when predicting the materials’ behavior at high temperatures. This parameter allows the dinstintion between those materials that lose their rigidity over the narrow range of temperatures (HDPE) and those which may support light loads at high temperatures.
1 Specific Volume
The Specific Volume of Polypropylene is the function of pressure and temperature. It is worth mentioning that the Density is the inverse of Specific Volume. There are status equations that predict accurately the PVT behavior of the Polypropylene over the melting point. Spencer and Gilmore equations are the most common ones: (P+1600)(V-0.620)=2*T in which P is the pressure in the atmospheres, V is the specific volume in cm³/gr and T is the melting temperature in Kelvin degrees.
That equation is very useful for predicting changes in the specific volume of material when there are pressure and temperature changes for the melting of PP. Below the melting point, the specific volume (density) is not determined only by pressure and temperature. The factors affecting the average crystallinity of the material (cooling rate, flow conditions, and so on) modify the specific volume and, thus, the density of semicrystalline materials below the melting temperature will not be only the function of pressure and temperature.
2 Environmental Resistance
PP has excellent mechanical properties at high temperatures. However, in order to take advantage of that, it is necessary to protect the material from the degradation produced by the oxidation during both the transformation and the lifetime of the finished product.
During the transformation, the material is subject to high temperatures and oxygen. The oxidation reaction takes place and it degrades the material. The degree of degradation shown with an increase of yield raises with the temperature. There also exists the long-term degradation which affects essentially the surface of the material, making it more brittle and giving place to mechanical failures.
So as to reduce these degradation reactions, during the production of CUYOLEN and CUYOTEC materials, two types of stabilizers are added: one acting during the transformation and the other enhancing the resistance to long term wear and tear.
There won’t be any problems with Cuyolen and Cuyotec degrees when common transformation conditions are employed. However, if the material is kept at the processing temperature during a long period, e.g. during a machine breakdown, there may be some problems.
When the polypropylene is exposed continuously outdoors, as some other polyolefins, it must be stabilized at UV light. The exposure to sunlight causes a gradual wear in the surface appareance. The degradation rate is increased with the raise of UV light intensity and the material’s temperature during the irradiation. Both factors change with latitude, altitude and seasons. The rain affects the degradation rate since it removes some types of stabilizers from the surface, leaving it susceptible to UV light.
3 Electrical Properties
For applications such as terminals, connectors, switches or cables sheaths, the electrical properties of Cuyolen and Cuyotec must be assessed, so that they behave as a good electrical insulator.
The basic properties that it has to meet are the following:
The dielectric rigidity of an insulating material is defined as the minimum voltage value to be applied to the insulating material necessary to make it fracture.
Polypropylene has typical dielectric rigidity of 140 Kv/mm stipulated, according to IEC 243 standard.
The dielectric constant (permittivity) is the capability of an insulating material to concentrate electric power.
The value of electric permittivity measured for PP is 2,3 to 1 Mhz according to IEC 250 standard.
It is a measure that states the amount of energy dissipated through the insulator when a certain voltage is applied to it.
The value of the dissipation factor for PP is 2 10-4 to 1 Mhz according to IEC 250 standard.
The main feature of an insulating material is its capability to oppose the flow of an electric current.
The two types of electric resistivity are volume and surface.
The volume resistivity is the resistance to current passage through the insulating material’s body.
Its measurement unit is ohm.m. The value of volume electric resistivity measured for Cuyolen is 1014 ohm.m according to IEC 93 standard.
The surface resistivity is the resistance to current passage through the insulating material’s surface.
Its measurement unit is ohm.
The value of surface electric resistivity measured for polypropylene is 1014 ohm according to IEC 93 standard.
The typical thermal properties of both polypropylene as raw material and finished products are as follows:
Softening Temperature: (BS2782: Part 1 Method 120A, ISO R306): 147-148°C
Crystalline Melting Point: 155-175°C
Autoignition Temperature: (ASTM D 1929): ca. 375°
Calorific Value: 46 KJ/g (11.000 cal/g)
Specific heat: 1.93 KJ/Kg (0.46 cal/g)
Thermal Conductivity: 0.21 W/m K (5x10-4 cal/seg cm °C)
Oxygen Index: (BS 2782 : (BS 2782 : Part 1 Method 141, ISO 4589, ASTM D 2863): 17.4 - 18%
Burning rate (horizontal): 18-25 mm/mi