Characterisation of the imbalance between various types of additives in PVC foamed sheets

1、Stabiliser

Polyvinyl chloride (PVC) has good acid and alkali resistance, abrasion resistance, electrical insulation, flame retardant self-extinguishing and low price and other advantages, is a very widely used thermoplastics. Although PVC has many of the above advantages, but has a fatal disadvantage, that is, poor thermal stability, PVC is usually heated to 140 ℃ when it begins to degrade, and its processing and moulding temperature in the 160-200 ℃ or so, therefore, the processing and moulding must be added to the heat stabilizer to inhibit the degradation of PVC. At present, the traditional heat stabiliser is mainly based on lead salts, organotin, metal soaps and other metal ion-containing heat stabilizers.

The role of heat stabilisers are.

1) neutralisation of HCl organic acids, metal soaps, some inorganic acids, alkali salts, epoxy compounds, amines, metal alcohol salts and phenols and metal thiol salts, etc. can be rapidly decomposed with PVC released HCl reaction, the generation of harmless compounds, and stabilising effect. Obviously, this type of stabiliser only through the acceptance and neutralisation of HCl to play an inhibitory role, and can not prevent the decomposition of PVC.

2) Substitution of unstable chlorine ship atoms Unstable chlorine atoms play an important role in the degradation process of PVC. Therefore, the most effective stabiliser should have a substitution reaction with unstable chlorine atoms, the weak link in the PVC molecule, before the decomposition of PVC occurs, so as to replace the unstable chlorine atoms, which are prone to degradation, with a more stable group to heat. Heavy metal hydroxylates and thiolates can play this role. However, further studies have shown that these salts also react with HCl, thus reducing the effective amount of substitution reaction; moreover, the heavy metal chlorides formed will promote the degradation of PVC. Therefore, the use of heavy metal hydroxylates and alkali metal or alkaline earth metal hydroxylates, epoxy compounds, synergistic effect to improve the stabilisation efficiency.

3) and unsaturated parts of the reaction with the use of a certain stabilizing group and the PVC chain of unsaturated double bond addition reaction, thereby inhibiting the dehydrogenation of HC1, and because of the conjugated polyene double Jian open and play a role in the decoloration, which is another effective stabilizing effect. For example, metal thiol salts react with HCf to form thiols, which can react with the double bond in an addition reaction to form a stable thioether bond on the PVC chain.

4) Tunnelling impurities A class of stabilizers such as phosphite ternary ester have the effect of neutralising or passivating certain resin impurities, trace metal contaminants, initiator residues, etc., so that the thermal stability of PVC resin is improved.

A single stabiliser is often not as good as to meet a variety of requirements, a variety of stabilizers are often used means of composite, the most ideal composite effect is to produce a synergistic effect.

The most widely used stabilisers for PVC foaming products are lead salt stabilisers and calcium-zinc stabilisers. As the requirements for environmental protection become more and more stringent, and the cost-effectiveness of calcium-zinc stabilisers becomes higher and higher, the application of calcium-zinc stabilisers starts to become more and more widespread. In the extrusion process of panels, especially wide panels, the molten material flows over long distances in the mould and stays there for a long time, which places high demands on the thermal stability and fluidity of the formulation, and as an outdoor user it must also be highly weather-resistant, so in addition to excellent long-term heat stabilizers, calcium-zinc stabilizers for panels also need to contain highly effective auxiliary heat stabilizers to inhibit the initial colouring and light stabilizers to provide weather-resistance.

Insufficient stabiliser, yellowing of the board surface, paste, brittleness of the product, reduced strength, low foaming rate;

If there is too much stabiliser, the foaming agent will decompose in advance, the gas will overflow from the charging holes and vacuum holes, and there will be cracks or shrinkage marks in the cavity structure;