Property Enhancement of PVC

The history of polyvinyl chloride (PVC) formulating is based on enhancing the properties derived from the starting raw material. The influence of impact modifiers and plasticisers has already been covered. This article focuses on work carried out to expand PVC, a so-called commodity plastic, into the property performance areas given by some engineering plastics, such as acrylonitrile–butadiene–styrene (ABS), and so on.

Crosslinked PVC

Crosslinking of PVC results in the improvement of a number of properties, particularly at elevated temperatures. Investigative work has been carried out on both unplasticised-PVC (PVC-U) and plasticised-PVC (PVC-P) formulations.

Chemical Crosslinking

Chemical crosslinkable formulations need to generate a sufficient crosslinked portion or gel content (up to 50%) with satisfactory processing stability (usually a conflict) and should not, of course, crosslink during processing. Crosslinking is achieved hydrolytically.

For some PVC-P formulations, mercaptosilanes and aminosilanes grafted by nucleophilic substitution during processing appear to be more suitable than organic peroxides in achieving improved solvent and abrasion resistance.

Organic peroxide crosslinking of PVC foam formulations (based on emulsion-grade resin) has shown the positive benefit of a trimethylolpropane trimethacrylate (TMPTMA) compound as co-agent, with good thermal stability and a very dense crosslink network. In the presence of the peroxide, radical reactions occur and the TMPTMA is grafted onto the PVC chains with a resultant network formation.

The use of a reactive plasticiser, triallyl cyanurate, has also been shown to improve the high temperature creep resistance of organic peroxide-crosslinked plastisols.

The chemical crosslinking of PVC-U formulations is considerably more difficult due to the fine balance of achieving a sufficient level of crosslinked polymer, adequate thermal stability, and the avoidance of premature crosslinking during processing. This has been investigated with successful results from aminosilane and peroxide/TMPTMA crosslinking systems, with slower crosslinking attributed to the reduced diffusion of water required to hydrolyse the silane. Thermal stability still has to be improved. The peroxide crosslinking system has shown gel contents of 30–40% with improved mechanical properties using appropriate curing agent levels.

Irradiation Crosslinking

Irradiation crosslinking of PVC-U carried out over a dosing range of 20–200 kGy using an electron beam has shown a marked increase in glass transition temperature (Tg) in the presence of trimethylolpropane triacrylate (TMPTA), with minimal thermal degradation. Electron beam irradiation has also been investigated on a PVC-P wire coating formulation, in the presence of different reactive monomers including TMPTA and TMPTMA, with the influence on different properties observed. Flame retardancy of PVC-P formulations for wire and cable has also been improved using TMPTA and irradiation doses of 90 and 120 kGy with the incorporation of appropriate flame retardants.

Orientation

The mechanical properties of PVC-U can be improved substantially by stretching the polymer, usually in both directions (biaxially) but may be in one direction (uniaxially), at temperatures above Tg. In addition, material costs are reduced. Uses to which this process can be applied are pipe and sheet.

Pipe

As already indicated in previous articles, pressure pipe design, based on conventional PVC-U, has to take account of a higher safety factor. This is due to the more brittle nature of PVC pipes in comparison to polyethylene (PE) and therefore having a greater risk of not performing to their strength potential.

If the ductile balance between strength and toughness could be changed to increase ductility, then a lower safety factor could be used. Biaxial PVC has both higher strength and toughness, probably justifying a lower safety factor.

However, the strength gains with oriented PVC are such that pipe stiffness limitations could be breached. This is a limiting criterion for strong materials where the pipe wall would be so thin as to allow deformation of the pipe. Thus, the safety factor of two has been maintained. Similarly, there is a limit to the strength that can be usefully exploited.

bellow Table shows the relationship between standard dimensional ratio (SDR; ratio between outside diameter and wall thickness) and stiffness for standard PVC-U and a biaxial PVC with a long term strength of 40 MPa and a safety factor of two. These values apply for a 1 MPa pipe.

A stiffness value of 4 kPa is the minimum allowable value in CEN pressure standards. In next Table, it can be seen how strength and safety factor may be combined while still meeting the stiffness criterion.

The process can be continuous and is essentially the extrusion of a thick-walled preform, which is stretched to produce a biaxial pipe. The key to the process is the mandrel over which the pipe is expanded, being hot and lubricated at the front end to facilitate the stretching process in the hoop direction. The orientation is frozen in at the back end of the mandrel which is cooled. A second calibration stage controls the outside diameter.

Molecular orientation introduces anisotropy into a material. This means having physical properties that are different in different directions. Thus, for effective enhancement of performance, properties must be enhanced in the appropriate direction and this is achieved in the process. There is a change in the stress yield curve with the removal of a clearly defined yield point.

Normally, an increase in strength is not conducive to an increase in both toughness and ductility. Brittle failures originate from defects that act as stress raisers. Although crack development and propagation is hindered by an increase in toughness, crack blunting by localised yielding is an effective part of the overall mechanism. It is the balance between strength and toughness that determines if brittle or ductile properties predominate. The enhanced toughness of biaxial pipe is shown by higher impact resistance and greater resistance to failure by slow crack creep mechanisms. This mechanism is based on the laminated structure of biaxial PVC, where crack propagation is hindered by the reduction in stress concentration at the crack tip as the crack passes through each layer.

Sheet

Biaxially orientated corrugated PVC sheets are produced for outdoor protection against the elements. Impact strength, including low-temperature impact, derived from the orientation process is significantly higher than that of standard extruded PVC corrugated sheet.

Blends and Alloys

Flexibilisers/Internal Plasticisers

In an attempt to move into application areas requiring the rubbery or elastomeric properties given

by thermoplastic elastomers (TPE) and thermoplastic olefi ns (TPO), the mechanical blending of PVC

resins with other elastomers has been developed. This internal plasticisation is the effect produced

by structural groups incorporated into a polymer, which also has the effect of lowering Tg. However,

cost effi ciency and ease of processing have to be considered.