Mechanical Properties in PVC

Mechanical properties are inevitably important properties, because most end uses involve mechanical loading under a particular service conditions. PVC, like other thermoplastics, is a viscoelastic material and mechanical properties depend on time, temperature, and stress. The infl uence of operating environment generally must be considered. Typical mechanical properties are shown as follows:

Hardness

Hardness is defined as the resistance of a material to deformation, in particular permanent deformation, indentation, or scratching. It is a relative term and should not be confused with wear and abrasion resistance.

The hardness test is used for measuring the relative hardness of soft materials and is based on the penetration of a specifi c indentor forced into the material under specified conditions. Two types of durometer are used, differing in the shape and dimension of the indentor. No units are attributed to hardness numbers. Shore A hardness is used for relatively soft materials and Shore D hardness for harder materials. The plasticiser content and type influence Shore A hardness.

The Rockwell hardness test (ISO 2039-2) may also be quoted for rigid PVC formulations. This measures the net increase in depth impression as the load on an indentor is increased from a fixed minor load to a major load and then returned to a minor load. Hardness numbers (without units), in increasing order of hardness, are R, M, and E scales for plastics. The higher the number in each scale, the harder the material. The different Rockwell hardness scales utilise different size steel balls and different loads. Rigid PVC has a Rockwell R hardness of 80–110 depending on the grade. This can be compared with PP at 90 R, polycarbonate (PC) at 124 R, ABS at 75–115 R, PS at 100 M, and PET at 196 M.

Tensile Properties

Tensile testing measures the ability of a material to withstand forces that tend to pull it apart and can determine to what extent the material stretches before breaking. Under laboratory conditions, tensile testing is carried out at a constant strain rate.

Once it has reached its high tensile strength, under a tensile load, rigid PVC flows in a plastic manner as the tensile stress is removed until plastic fracture occurs.

Tensile modulus (Young’s modulus) is the force that is needed to elongate the material. For PVC-U, the tensile modulus is in the region of 3.5 GPa. The 100% modulus is the stress/strain ratio at 100% extension. In PVC-P formulations, this is influenced by plasticiser content and type.

shows the relationship between 100% modulus and Shore A hardness.

Flexural Properties

Flexural property measurement measures the stress–strain behaviour in bending mode. Flexural strength is the ability of a material to withstand bending forces applied perpendicular to its longitudinal axis. The stresses induced are a combination of compressive and tensile stresses.

Flexural modulus is a measure of the stiffness during the initial part of the bending process. Figures 4.4 and 4.5 show the infl uence of temperature on fl exural modulus. As expected, the influence of an impact modifi er reduces fl exural modulus. The superior fl exural modulus at higher temperatures for PVC-C can also be seen. Increasing plasticiser content has the effect of reducing flexural modulus.

Impact Properties

Impact properties are related to material toughness, with toughness defined as the ability of PVC to absorb applied energy. Impact resistance is the ability to resist breaking under shock loading or being able to resist fracture under stress applied at high speed.

The use of a pendulum impact tester is common (set up for the different striker heads and sample supports appropriate for Charpy, Izod, or tensile impact test methods) with results expressed in terms of kinetic energy consumed by the pendulum in order to break the specimen. The test result is typically the average for 5 or 10 test specimens. Charpy or Izod tests are the most common tests with impact-modifi ed PVC.

The specimen is usually notched with the shape of the notch usually being a V-cut, but may be a U-cut. Notch depth and geometry can vary depending on the particular test method. Notching provides a stress concentration area that promotes a failure. The mode of failure should also be noted in relation to brittle (fractures without yielding) and ductile (material yields in addition to cracking). Notched impact values of unmodified rigid PVC are signifi cantly lower due to notch sensitivity. Impact strength is still good for unmodified PVC, providing surface notches are avoided. Notched impact testing is used extensively as an economical quality control method to assess the notch sensitivity and impact toughness. The precision of the cutting of the notch is critical and special notch cutting equipment is available to give accurate and reproducible results. The Izod impact test (ASTM D256 and ISO 180), most common in the USA, utilities a pendulum which swings on its track and strikes a notched sample, secured at one end, on the thickness

where the notch has been made. The energy lost, i.e., required to break the sample, is measured from the distance the pendulum swings after impact. The result is reported in energy lost per unit of specimen thickness (J/cm) for the ASTM method or energy lost per unit cross-sectional area at the notch (J/m2) for the ISO method, at a standard temperature (normally 23 °C). The minimum requirement for window profi le, in the USA and using this test method, is 5.3 kJ/m2 when measured under the ISO method.

The single V-notch Charpy test (BS EN ISO 179, 0.1 mm notch radius) has been common in the UK and relies on a horizontally mounted sample which is supported unclamped at both ends. The pendulum hammer (of fixed kinetic energy) strikes the sample on the thickness opposite from the notched side. The crack propagates on the opposite side from the impact. The minimum accepted figure for this method is 12 kJ/m2 for window profi le. Brittle failure is common with this test method.

ISO 179-1 (0.25 mm notch radius) single notch Charpy test can also be used.

In the double V-notched Charpy test (ISO 179-1), commonly used in Germany, and results quoted in the first Table, a V-notch is cut into opposite edges of the test sample with the notches precisely across from each other. The pendulum hits the face of the horizontal specimen between the notches, with crack propagation occurring at a right angle to the impact location. A ductile behaviour results when impact tested. The requirement is a minimum of 40 kJ/m2 for window profile. This test is also used

to check impact retention after artificial weathering/ageing. The new European Standard for PVC-U window profiles, EN 12608, has now imposed a new Charpy test, the ISO 179-2 (0.25 mm single-V notch), to quantify the impact resistance. A double-V

notch Charpy test, with a notch radius of 0.25 mm, is also specified in this standard for the assessment of the impact strength retention of extruded profile after artificial weathering. The test must be performed using instrumented equipment, on samples cut from a 4 mm thick, milled and pressed plate. This plate can be prepared from either shredded profile or directly from the dry blend or compound. For impact modified material, the minimum acceptable level is 20 kJ/m2. There is considerable debate that such an impact test takes no account of processing variables or profile thickness effects.

Fatigue

Fatigue is defined as a decline in load-bearing capacity with time under load. Under conditions of constant load, this is termed static fatigue or creep rupture. On average, the long-term allowable stress of a plastics product, operating at ambient temperatures, is no greater than one-fifth the ultimate short-term strength of the material. As the tensile strengths of unreinforced thermoplastics mainly fall within the range 20–70 MPa, this would suggest an ambient temperature range of 4–14 MPa for allowable or safe stresses. A useful body of data exists for thermoplastic piping materials, which have been subjected to long-term pressure testing at ambient and elevated temperatures. The safe allowable static tensile (hoop) stresses for 100,000 hours (11.4 years) service are shown in following table.

Dynamic fatigue, which is the durability of plastics under cyclic loading, is generally less than under static loading, with amorphous thermoplastics, such as PVC-U, being particularly sensitive. Crack growth rates increase by a factor of 5,000 under cyclic loading with decreasing molecular weight (200,000–500,000).