Biaxial oriented PVC (PVCO) is a form of PVC pipe where the pipe wall consists of many layers, created through a stretching process. This multilayer structure acts effectively as a crack stopper, significantly improving the pipe's fracture resistance.
The stretching process, which involves orientation in both axial (lengthwise) and tangential (circumferential) directions, results in a material with a much higher strength compared to standard un-plasticised PVC (UPVC). The Minimum Required Strength (MRS) of biaxial stretched PVC can be around 40-50 MPa, depending on the stretching conditions.
The increased MRS of PVCO allows for a lower safety factor of 1.6, compared to the 2-2.5 factor typically used for UPVC. When combined in the pressure rating formula (PN), this higher strength and lower safety factor result in a design strength that has more than doubled for MRS 450.
Consequently, for the same pressure class, the wall thickness of OPVCO pipe is less than half that of PVCU. This reduction in material usage leads to significant benefits: at the same production output rate, the produced length of PVCO pipe is more than doubled compared to UPVC. This efficiency gain also contributes positively to sustainability. PVCO exhibits a straight regression line in lifetime graphs without the 'knee' seen in some other polymers like PE, indicating predictable long-term behavior.
Commercial production of PVCO began in the UK in the 1980s using a mould system where a preform was blown up in diameter. While mould systems are still used (e.g., by Molecor in Spain), they can have disadvantages like lack of axial stretching, lost pipe ends, and wide wall distribution.
More modern approaches utilize inline systems, such as those developed by Vinidex (Australia) and Wavin (Europe), or the latest dry in-line Biax process (Rollepaal PVC-O). These inline methods offer better control over the process and pipe dimensions.
Standards for PVCO include AS/NZS 4441, ASTM F1438, and ISO 16422. Safety factors are determined according to standards like ISO 12162.
|
Property |
PVCO Type |
Value |
Standard/Context |
|
MRS |
Biaxial Stretched PVC |
40-50 MPa |
Depending on stretching cond. |
|
Safety Factor |
PVCO |
1.6 |
Relative to PN calculation |
|
Wall Thickness |
PVCO vs UPVC |
Less than half |
For the same pressure class |