Polyvinyl Chloride (PVC)
Polyvinyl chloride is a thermoplastics material which consists of PVC resin compounded with varying proportions of stabilisers, lubricants, fillers, pigments, plasticisers and processing aids. Different compounds of these ingredients have been developed to obtain specific groups of properties for different applications. However, the major part of each compound is PVC resin.
The technical terminology for PVC in organic chemistry is poly (vinyl chloride): a polymer, i.e. chained molecules, of vinyl chloride. The brackets are not used in common literature and the name is commonly abbreviated to PVC. Where the discussion refers to a specific type of PVC pipe, that type will be explicitly identified as detailed below. Where the discussion is general, the term “PVC pipes” will be used to cover the range of PVC pipe pressure materials supplied by Vinidex.
Different Types of Polyvinyl Chloride
The PVC compounds with the greatest short-term and long-term strengths are those that contain no plasticisers and the minimum of compounding ingredients. This type of PVC is known as UPVC or PVC-U. Other resins or modifiers (such as ABS, CPE or acrylics) may be added to UPVC to produce compounds with improved impact resistance. These compounds are known as modified PVC (PVC-M). Flexible or plasticised PVC compounds, with a wide range of properties, can also be produced by the addition of plasticisers. Other types of PVC are called CPVC (PVC-C) (chlorinated PVC), which has a higher chlorine content and oriented PVC (PVC-O) which is PVC-U where the molecules are preferentially aligned in a particular direction.
PVC-U (unplasticised) is hard and rigid with an ultimate tensile stress of approximately 52 MPa at 20°C and is resistant to most chemicals. Generally PVC-U can be used at temperatures up to 60°C, although the actual temperature limit is dependent on stress and environmental conditions.
PVC-M (modified) is rigid and has improved toughness, particularly in impact. The elastic modulus, yield stress and ultimate tensile strength are generally lower than PVC-U. These properties depend on the type and amount of modifier used.
PVC (plasticised) is less rigid; has high impact strength; is easier to extrude or mould; has lower temperature resistance; is less resistant to chemicals, and usually has lower ultimate tensile strength. The variability from compound to compound in plasticised PVC is greater than that in PVC-U. Vinidex does not manufacture pressure pipes using plasticised PVC.
PVC-C (chlorinated) is similar to PVC-U in most of its properties but it has a higher temperature resistance, being able to function up to 95°C. It has a similar ultimate stress at 20°C and an ultimate tensile stress of about 15 MPa at 80°C.
PVC-O (Oriented PVC) is sometimes called HSPVC (high strength PVC). PVC-O pipes represent a major advancement in the technology of the PVC pipe industry.
PVC-O is manufactured by a process which results in a preferential orientation of the long chain PVC molecules in the circumferential or hoop direction. This provides a marked enhancement of properties in this direction. In addition to other benefits, ultimate tensile strength up to double that of PVC-U can be obtained for PVC-O. In applications such as pressure pipes, where well defined stress directionality is present, very significant gains in strength and/or savings in materials can be made.
Typical properties of PVC-O in the hoop direction are:
- Tensile Strength of PVC-O – 90 MPa
- Elastic Modulus of PVC-O – 4000 MPa
Property enhancement by molecular orientation is well known and some industrial examples have been produced for over thirty years. In more recent times, it has been applied to consumer products such as films, high strength garbage bags, carbonated beverage bottles and the like.
The technique for applying molecular orientation to PVC pipes was pioneered during the 1970’s by Yorkshire Imperial Plastics and in fact the earliest trial installations were made in 1974 with 100 mm pipe by the Yorkshire Water Authority, United Kingdom. Vinidex commenced production in a pilot PVC-O pipe plant in early 1982 and PVC-O pipes were first installed in Australia in 1986. Since that time, Vinidex have continued to develop and expand the PVC-O product range in commercial production under the trade name Supermain.
Comparison between PVC-O, PVC-M and Standard PVC-U
PVC-O is identical in composition to PVC-U and their general properties are correspondingly similar. The major difference lies in the mechanical properties in the direction of orientation. The composition of PVC-M differs by the addition of an impact modifier and the properties deviate from standard PVC-U depending on the type and amount of modifier used. The following comparison is general in nature and serves to highlight typical differences between pipe grade materials.
Tensile Strength – The tensile strength of PVC-O is up to twice that of normal PVC-U. The tensile strength of PVC-M is slightly lower than standard PVC-U.
Toughness – Both PVC-O and PVC-M behave in a consistently ductile manner under all practical circumstances. Under some adverse conditions, in the presence of a notch or flaw, standard PVC-U can exhibit brittle characteristics.
Safety Factors – Design of PVC pipes for pressure applications involves prediction of long term properties and application of a safety factor. As in all engineering design, the magnitude of the safety factor reflects the level of confidence in the prediction of performance. The greater confidence in predictable behaviour for the new generation materials PVC-M and PVC-O has the benefit of allowing a lower factor of safety to be used in design.
Design Stress – PVC-O and PVC-M pipes operate at a higher design stress than standard PVC-U pipes as a result of their reduced safety factor and in the case of PVC-O, higher strength in the hoop direction.
Elasticity and Creep – PVC-O has a modulus of elasticity up to 24% higher than normal PVC-U in the oriented direction and a similar modulus to standard PVC-U in other directions. The elastic modulus of PVC-M is marginally lower than standard PVC-U.
Impact Characteristics – PVC-O exceeds standard PVC-U by a factor of at least 2 and up to 5. PVC-M also has greater impact resistance than standard PVC-U. Impact performance tests for PVC-M pipes focus on obtaining a ductile failure characteristic.
Weathering – There are no significant differences in the weathering characteristics of PVC-U, PVC-M and PVC-O.
Jointing – PVC-U and PVC-M pipes can be jointed by either rubber ring or solvent cement joints. PVC-O is available in rubber-ring jointed pipes only. PVC-O cannot be solvent-cement jointed.
Properties of PVC
General properties of PVC compounds used in pipe manufacture are given in the Table below. Unless otherwise noted, the values given are for standard unmodified formulations using K67 PVC resin. Some comparative values are shown for other pipe materials. Properties of thermoplastics are subject to significant changes with temperature, and the applicable range is noted where appropriate. Mechanical properties are subject to duration of stress application, and are more properly defined by creep functions. More detailed data pertinent to pipe applications are given in the design section of this manual. For data outside of the range of conditions listed, users are advised to contact our Technical Department.
Typical Properties of PVC Pipe Material
|Property||Value||Conditions and Remarks|
|Molecular weight (resin)||140000||cf: K57 PVC 70,000|
|Relative density||1.42 – 1.48||cf: PE 0.95 – 0.96, GRP 1.4 – 2.1, CI 7.2, Clay 1.8 – 2.6|
|Water absorption||0.0012||23°C, 24 hours cf: AC 18 – 20% AS1711|
|Hardness||80||Shore D Durometer, Brinell 15, Rockwell R 114, cf: PE Shore D 60|
|Impact strength – 20°C||20 kJ/m2||Charpy 250 µm notch tip radius|
|Impact strength – 0°C||8 kJ/m2||Charpy 250 µm notch tip radius|
|Coefficient of friction||0.4||PVC to PVC cf: PE 0.25, PA 0.3|
|Property||Value||Conditions and Remarks|
|Ultimate tensile strength||52 MPa||AS 1175 Tensometer at constant strain rate cf: PE 30|
|Elongation at break||50 – 80%||AS 1175 Tensometer at constant strain rate cf: PE 600-900|
|Short term creep rupture||44 MPa||Constant load 1 hour value cf: PE 14, ABS 25|
|Long term creep rupture||28 MPa||Constant load extrapolated 50 year value cf: PE 8-12|
|Elastic tensile modulus||3.0 – 3.3 GPa||1% strain at 100 seconds cf: PE 0.9-1.2|
|Elastic flexural modulus||2.7 – 3.0 GPa||1% strain at 100 seconds cf: PE 0.7-0.9|
|Long term creep modulus||0.9 – 1.2 GPa||Constant load extrapolated 50 year secant value cf: PE 0.2 – 0.3|
|Shear modulus||1.0 GPa||1% strain at 100 seconds G=E/2/(1+µ) cf: PE 0.2|
|Bulk modulus||4.7 GPa||1% strain at 100 seconds K=E/3/(1-2µ) cf: PE 2.0|
|Poisson’s ratio||0.4||Increases marginally with time under load. cf: PE 0.45|
|Property||Value||Conditions and Remarks|
|Dielectric strength (breakdown)||14 – 20 kV/mm||Short term, 3 mm specimen cf PE 70 – 85|
|Volume resistivity||2 x 1014 Ω.m||AS 1255.1 PE > 1016|
|Surface resistivity||1013 – 1014 Ω||AS 1255.1 PE > 1013|
|Dielectric constant (permittivity)||3.9 (3.3)||50 Hz (106 Hz) AS 1255.4 cf PE 2.3 – 2.5|
|Dissipation factor (power factor)||0.01 (0.02)||50 Hz (106 Hz) AS 1255.4|
|Property||Value||Conditions and Remarks|
|Softening point||80 – 84°C||Vicat method AS 1462.5 (min. 75°C for pipes)|
|Max. continuous service temp.||60°C||cf: PE 80*, PP 110* not under pressure|
|Coefficient of thermal expansion||7 x 10-5 K||7 mm per 10 m per 10°C cf: PE 18 – 20 x 10-5, DI 1.2 x 10-5|
|Thermal conductivity||0.16 W/(m.K)||0 – 50°C PE 0.4|
|Specific heat||1,000 J/(kg.K)||0 – 50°C|
|Thermal diffusivity||1.1 x 10-7 m2/s||0 – 50°C|
|Property||Value||Conditions and Remarks|
|Flammability (oxygen index)||0.45||ASTM D2863 Fennimore Martin test, cf: PE 17.5, PP 17.5|
|Ignitability index||10 – 12 (/20)||cf: 9 – 10 when tested as pipe AS 1530 Early Fire Hazard Test|
|Smoke produced index||6 – 8 (/l0)||cf: 4 – 6 when tested as pipe AS 1530 Early Fire Hazard Test|
|Heat evolved index||0|
|Spread of flame index||0||Will not support combustion. AS 1530 Early Fire Hazard Test|
- PE: Polyethylene
- PP: Polypropylene
- PA: Polyamide (nylon)
- CI: Cast Iron
- AC: Asbestos Cement
- GRP: Glass Reinforced Pipe
Conversion of Units
- 1MPa = 10bar = 9.81 kg/cm2 = 145lbf in2
- 1 Joule = 4.186 calories = 0.948 x 10-3 BTU = 0.737 ft.lbf
- 1 Kelvin = 1°C = 1.8°F temperature differential
For PVC, like other thermoplastics materials, the stress /strain response is dependent on both time and temperature. When a constant static load is applied to a plastics material, the resultant strain behaviour is rather complex. There is an immediate elastic response, which is fully recovered as soon as the load is removed. In addition there is a slower deformation, which continues indefinitely while the load is applied until rupture occurs. This is known as creep. If the load is removed before failure, the recovery of the original dimensions occurs gradually over time. The rate of creep and recovery is also influenced by temperature. At higher temperatures, creep rates tend to increase. Because of this type of response, plastics are known as viscoelastic materials.
The Stress Regression Line
The consequence of creep is that pipes subjected to higher stresses will fail in a shorter time than those subjected to lower stresses. For pressure pipe applications, long life is an essential requirement. Therefore, it is important that pipes are designed to operate at wall stresses which will ensure that long service lives can be achieved. To establish the long term properties, a large number of test specimens, in pipe form, are tested until rupture. All of these separate data points are then plotted on a graph and a regression analysis performed. The linear regression analysis is extrapolated to obtain the 97.5% lower prediction limit failure stress at the design point which must exceed a minimum required stress (MRS).
A safety factor is then applied to the MRS to obtain a maximum operating stress for the pipe material which is used to dimension pipes for a range of pressure ratings. In Europe and Australasia, the ISO design point of 50 years, or 438,000 hours, is adopted. In North America, the design point of 100,000 hours has historically been used. This design point is quite arbitrary and should not be interpreted as an indication of the expected service life of a PVC pipe. The stress regression line is traditionally plotted on logarithmic axes showing the circumferential or hoop stress versus time to rupture.
*For PVC-M and PVC-O, the 50-year specification point is a 97.5% lower confidence limit point to ensure that the minimum factor of safety is obtained.
For PVC, the modulus or stress/strain relationship must be considered in the context of the rate or duration of loading and the temperature.
A universal method of data presentation is a curve of strain versus time at constant stress. At a given temperature, a series of curves is required at different stress levels to represent the complete picture. A modulus can be computed for any stress/strain/ time combination, and this is normally referred to as the creep modulus.
Such curves are useful, for example, in designing for short and long term transverse loadings of pipes.
Tests conducted in both England and Australia have shown that PVC-O is stiffer, i.e. it has a higher modulus, than standard PVC-U by some 24% for equivalent conditions in the oriented direction. From other work, there appears to be no significant change in the axial direction.
Pressure Ratings at Elevated Temperatures
The mechanical properties of PVC are referenced at 20°C. Thermoplastics generally decrease in strength and increase in ductility as the temperature rises and design stresses must be adjusted accordingly.
The term “reversion” refers to dimensional change in plastics products as a consequence of “material memory”. Plastics products “memorise” their original formed shape and if they are subsequently distorted, they will return to their original shape under heat.
In reality, reversion proceeds at all temperatures, but with high quality extrusion it is of no practical significance in plain pipe at temperatures below 60°C and in PVC-O pipe at temperatures below 50°C.
Weathering and Solar Degradation
The effect of “weathering” or surface degradation by radiant energy, in conjunction with the elements, on plastics has been well researched and documented. Solar radiation causes changes in the molecular structure of polymeric materials, including PVC. Inhibitors and reflectants are normally incorporated in the material which limits the process to a surface effect. Loss of gloss and discolouration under severe weathering will be observed. The processes require input of energy and cannot proceed if the material is shielded, e.g. under-ground pipes. From a practical point of view, the bulk material is unaffected and performance under primary tests will show no change, i.e. tensile strength and modulus. However, microscopic disruptions on a weathered surface can initiate fracture under conditions of extreme local stress, e.g. impact on the outside surface. Impact strength will therefore show a decrease under test.
Protection against Solar Degradation
All PVC pipes manufactured by Vinidex contain protective systems that will ensure against detrimental effects for normal periods of storage and installation. For periods of storage longer than one year, and to the extent that impact resistance is important to the particular installation, additional protection may be considered advisable. This may be provided by under-cover storage, or by covering pipe stacks with an appropriate material such as hessian. Heat entrapment should be avoided and ventilation provided. Black plastic sheeting should not be used. Above-ground pressure pipe systems may be protected by a coat of white or pastel-shade PVA paint. Good adhesion will be achieved with simply a detergent wash to remove any grease and dirt.
The ultimate strength of PVC does not alter markedly with age. Its short-term ultimate tensile strength generally shows a slight increase. It is important to appreciate that the stress regression line does not represent a weakening of the material with time, i.e. a pipe held under continuous pressure for many years will still show the same short-term ultimate burst pressure as a new pipe. The material does, however, undergo a change in morphology with time, in that the “free volume” in the matrix reduces, with an increasing number of cross-links between molecules. This results in some changes in mechanical properties:
- A marginal increase in ultimate tensile strength
- A significant increase in yield stress
- An increase in modulus at high strain levels
In general, these changes would appear to be beneficial. However, the response of the material at high stress levels is altered in that local yielding at stress concentrators is inhibited, and strain capability of the article is decreased. Brittle-type fracture is more likely to occur, and a general reduction in impact resistance may be observed.
These changes occur exponentially with time, rapidly immediately following forming, and more and more slowly as time proceeds. By the time the article is put into service, they are barely measurable, except in the very long term. Artificial ageing can be achieved by heat treatment at 60°C for 18 hours. PVC-O undergoes such ageing in the orientation process and its characteristics are similar to a fully aged material, but with greatly enhanced ultimate strength.
Plastics generally show excellent performance under abrasive conditions. The main properties contributing to this are the low elastic modulus and coefficient of friction. This enables the material to “give” and particles tend to skid rather than abrade the surface.
Well known low friction materials such as Teflon, Nylon and Polyurethanes show outstanding characteristics. Economics, however, are a major factor and PVC’s performance in the context of wear rate/unit cost is excellent. Factors affecting abrasion are complex and it is difficult to relate test data to practical conditions.
The Institute for Hydromechanic and Hydraulic Structures of the Technical University of Darmstadt in West Germany tested the abrasion resistance of several pipe products. Gravel and river sand were the abrasive materials used in concrete pipe, glazed vitrified clay pipe and PVC piping, with the following results:
|Measurable wear at 150,000 cycles|
|Minimal wear at 260,000 cycles. Accelerated wear after glazing wore off at 260,000 cycles.|
|PVC||Minimal wear at 260,000 cycles (about equal to glazed vitrified clay, but less accelerated than vitrified clay after 260,000 cycles)|
PVC is immune to attack by microbiological organisms normally encountered in under-ground water supply and sewerage systems.
PVC does not constitute a food source and is highly resistant to damage by termites and rodents.
Effect of Soil Sulphides
Grey discolouration of under-ground PVC pipes may be observed in the presence of sulphides commonly found in soils containing organic materials. This is due to a reaction with the stabiliser systems used in processing. It is a surface effect, and in no way impairs performance.