GTS PVC Pool

Following very profounding and practice-orientated information are only for registered members. You have the possibility to insert your PVC-knowledge. Your contributions will be integrated at an appropriate part by naming the author. All know-how which has not been marked is solely know-how of EJKrüger GmbH or GTS.

Formulation components

Introduction
PVC can only be processed with the aid of stabilisers, lubricants and other additives. Additionally, colours, fillers, different plasticisers as well as other additives are used. Additives, in particular the stabilisers and lubricants, frequently have a synergistic behaviour. All formulation components are based on 100 pph PVC.

The formulation components differ basically for pipe and profile extrusion, plasticised PVC, for the PVC injection moulding and for calendering. In particular the “environmental discussion“ held in the years 1988 to 1993 have had a very strong influence in the fact that Cadmium (Cd) has disappeared nearly completely from the today’s formulations.

Lubricants are used to decrease the internal and outside friction resistances.

Thermal stabilisers should catch decomposition products which result from thermal degradation. Light stabilisers have similar functions which can absorb or reflect light and bind the decomposition products.

PVC – basic formulation components

a) Stabilisers
Lead (Pb) can bind the chlorine (Cl2) or hydrogen chloride (HCl) which is decomposed from the PVC. In addition the following chemical compounds can be used
Lead stabilisers
Pb-content in mass-%
name formulation total active free
white lead Pb(OH)22PbCO3 80,2 80,2 26,7
alkali lead carbonate 2 PbOPbCO3 87,1 87,1 58,1
tri alkali lead sulphate 3 PbOPbSO4H2O 83,6 62,8 62,8
tetra alkali lead sulphate 4 PbOPbSO41/4 H2O 86,4 69,0 69,0
di alkali lead phosphite 2 PbOPbHPO31/2 H2O 83,7 83,7 55,8
di alkali lead stearate 2 PbOPb(O2C-C17H35)2 51,0 51,0 34,0
neutral lead stearate Pb(O2C-C17H35)2 27,0 27,0 0
di alkali lead phthalate 2 PbOPb(O2C)2C6H4 76,0 76,0 50,6
tri alkali lead maleate 3 PbOPb(O4C4H2)H2O 82,1 82,1 61,6
lead salicylate Pb[C6H4(OH)COO] 2 43,1 43,1 0
lead silicate PbO2 SiO2H2O 57,3 57,3 0
Firmenschrift Bleistabilisatoren von Interstab
In the column “total” the entire lead content is given. In the column “active” the quantity of lead is given which is able to bind hydrogen chloride. The column “free” contains the percentage lead, which is present as lead oxide or hydroxide.
The figures are described by the example of the alkali lead carbonate:
The molecular masses are: Pb = 207 g/mol, O = 16 g/mol, C = 12 g/mol.
Thus the mixture 2 PbOPbCO3 has a molecular mass of 713 g/mol.
Altogether 3 mol lead are in the compound = 3 * 207 / 713 = 87.1 %.
As lead oxide are present 2 mol lead = 2 * 207 / 713 = 58.1 %.

The amount of the free lead is an important value for the estimation of the necessary stabiliser quantity. E.g., if the lead content of white lead is also calculated into the active part of the lead, then it can occur that the lead carbonate also reacts with the chorine or HCl. The liberated carbon dioxide leads to gas bubbles in the melt. The use of alkali lead carbonate is here much safer, since the free lead content is more than twice as much.

Lead sulphate: is a thermal stabiliser and is torque neutral.
Lead phosphite: is a good thermal and light stabiliser works, however, torque-increasing.
General aspects:
• thermal stability rises with rising Pb-content
• 3 PbO is usual, 4 PbO is better but more expensive
• neutral lead stearate with 27 % lead is usual; this has, however, a slipping effect
• di akali lead stearate with 51% lead has a higher thermal stability; this has, however, a torque-increasing effect.

b) Lubricants
The correct selection of lubricants for the processing of rigid PVC is similarly important than the thermal stabilisation or the PVC type. Because of the great number of lubricants this topic can only be treated generally. Lubricants are divided into external and internal lubricants. External lubricants are defined as materials which reduce the friction between melt and metal surface. Most lubricants act, however, in both directions as shown in the following table.

Remarks:
• Lead stearate and stearic acid have a clearly synergistic effect.
• Use PE waxes very carefully, since they have a very large influence on the slipping properties. Must be increased e.g. if the melt at the screw tip is strongly warmed up and begin to form visible flow lines.
• For window formulations lead phosphite is used as the only stabiliser.
There are two basic stabilisation philosophies for lead:
1. Pb-salts + lubricants + stearates: is usual and recommended
2. Pb-stearates + lubricants: is cheaper, however, disadvantages in cold weather and during recycling can occur (absolute value from the lead is to small, with temperature changes, the stabiliser and lubricants show migration).

slip agents lubricating effect
long chain alcohol, poly-alcohol

dicarbonic acid-esters
glycerine ester
metal soaps / Ca-stearate
fatty acid esters
neutral Lead – stearate internal
fatty acid amides
fatty acids
ester waxes C28 – 32 external
acid waxes C28 – 32
PE-wax, oxidised
paraffin, liquid
paraffin, solid
polyethylene

Tests for sufficient stability
1. test: four times extrusion and recycling. If the pipe is okay, than the stabilisation is correct.
2. Thermal stability after the extrusion at 200 °C with indicator paper
pipe formulation 10 – 15 min
window formulation 15 – 30 min

c) Enhanced impact strength for PVC
Bad impact strength properties are observed whenever glass transition temperatures are below the utilisation temperatures. By the use of elastomer materials the range of application for the brittle material PVC is clearly enlarged.
Impact resistant PVC types can be produced in different ways:
• Powder-compounding: PVC and the elastomer component are mixed as powders to a Polyblend.
• Melt-compounding: The PVC-elastomer-mixture and other additives are melted and processed to a compound.
• Graft copolymerisation: The elastomer is added to the vinyl chloride monomer during the polymerisation of PVC and is bound to the molecule chain. Thus separation can be avoided which can occur with powder compounding. Graft copolymers have good free-flowing properties.
The crack propagation is diverted by the flexible elastomers, so that a part of the energy can be converted into heat. Mixtures of impact modifier can be combined to give a broad processing window with an acceptable efficiency.
The processing temperature and residence time have an influence on the formation of the elastomer network and achieve e.g. an optimum for EVAC with 190 °C and 5 min. milling time.

d) Light stabilisers
Light and oxygen destroy the molecule chains of most plastic materials and give worse mechanical and optical characteristics. A basic stabilisation is already achieved for PVC by the thermal stabilisation (e.g. lead phosphite). Light stabilisers work in 3 different mechanisms:
1. Absorption: convert light energy into heat energy.
2. Quencher: change energy of activated molecules in to heat or harmless radiation
3. Radical inhibitors: form harmless radicals, which react with harmful radicals (e.g. Cl•) and inhibit them.
Depending upon the climate zone, different demands are necessary for the light stabilisation. A good UV absorber is titanium dioxide in the rutile form. This pigment absorbs more than 40 % of the light with a wavelength smaller than 400 nm. Similar effect can be achieved with benzophenone derivatives.
The light stabilisation is better, the more completely the different decomposition mechanisms are prevented. Therefore synergistic effects can be observed on mixtures of different stabilisers.

e) Fillers
Fillers are additives which differ from the plastic matrix according to composition and or structure. They are usually inorganic, more rarely organic fillers. So called “extender” or inactive fillers are used to reduce the price of the formulation. Active fillers are used in order to improve certain mechanical or physical properties. Particularly the application of chalk is usual with the processing of PVC, since the impact strength can be improved and additionally the price of the formulation is cheaper. Chalk has, depending upon particle diameter, around 10 to 100 times larger surface area than PVC. Therefore surface effects can be observed during the production of the dry-blend. This aspect will be discussed more thoroughly in the section “Mixing”.

f) Processing and flowing aids
A further special property of PVC is the effect of wall slipping. Most other plastics are wall adhesive. Therefore processing or flowing aids are added to PVC in order to
• shorten the plasticising process
• modify the rheological properties and
• improve the mechanical properties in the solid state.

g) Plasticisers
Plasticisers are added to lower the fusing temperature, the modulus of elasticity and the glass transition temperature. There are so-called external plasticisers which influence the degree of softness depending upon type and concentration. They do not change the macromolecules themselves. The extractability of the plasticiser has disadvantages in some special cases. On the other hand the principle of the internal plasticiser is based on a modification of the molecular structure, as it can be obtained by co-polymerisation. Each co-polymer has only a small application window. Thus external plasticising is more important than internal plasticising.
The migration behaviour of the plasticiser is connected with it’s molecular size. The larger the molecule, the smaller is the tendency of migration.