"Cross-linking"
Crosslinking is the core chemical process of linking the plastic rubber molecules into a three-dimensional network structure with elastic properties, namely the finished rubber. The choice of crosslinking agent, the desired crosslinking density and the reactivity of the crosslinking system used have a decisive influence on the material properties. As a result, there may be substantial changes in their stress values, tensile strength, hardness, elasticity, gas permeability, high-temperature or swelling resistance during the crosslinking reaction.
The most well-known crosslinking agent is elemental sulfur, which is used in conjunction with zinc oxide, stearic acid and compounds known as vulcanization accelerators. In addition, sulphur-free systems are used as well, such as p-quinone dioxime together with oxidizing agents, peroxides with crosslinking coagents, diamino compounds, resins or metal oxides.
The choice of crosslinking systems is determined on the one hand by the chemical characteristics of the polymer. For instance, rubbers containing diene groups, such as NR, IR, SBR, BR or EPDM, can be crosslinked with numerous versions of the classical sulphur system. However, similar attempts with EVA, AEM or FKM would be doomed to failure.
On the other hand, the method intended for processing the rubber blend also plays a role. A rubber blend with a vulcanization system that is geared towards prompt crosslinking in a profile extrusion method would result in premature scorching, incomplete mould-filling and blockage of the injection channels. The geometry of the parts also has to be taken into account. Thick-walled rubber articles, for instance, require different systems to achieve the same level of crosslinking throughout the material as compared with thin diaphragms or windscreen wiper blades.
Furthermore, the choice of crosslinking system also determines the subsequent properties of the vulcanizate such as heat stability, permanent set characteristics, tear propagation and dynamic attributes.
Hence if the acceleration and crosslinking involve an elemental chemical reaction with a complex mechanism, only a few phr of the overall formulation will be involved. These minimum constituents have to be distributed as evenly as possible throughout the entire volume of the blend – batch for batch.
LUVOMAXX® solutions
We offer you the option of designing the discontinuous mixing process to make it reproducible, clean and cost-efficient. Suitable methods include
- predispersed polymer-bound accelerators
- liquid accelerators in the form of dry liquids
- oil-coated accelerators
Instead of the pure powders or liquids, polymer-bound preparations or dry liquids avoid material losses and deposits on the weighing equipment or in the feed chute of the internal mixer. The predispersed form further enhances the chemical activity of the active ingredients so that in many cases the quantity of material is reduced. Predispersed, strained crosslinking agents and accelerator masterbatches also avoid unwanted surface defects or uneven crosslinking, thus reducing the scrap rate.
During “acceleration” on the roll mill some additives in pure powder form behave almost like release agents and absolutely refuse to bind with the sheet. The result is material losses to the extraction system or catch pan. However, this is not the case with polymer-bound accelerators or oil-coated accelerator pastes. Here paste-like zones form in no time at all on the sheet and these are immediately distributed and incorporated. The low-dust accelerator is placed where it belongs: in the blend.
Combags: Stirred – not shaken!
Combags are combinations of different powdered accelerators and other additives combined in accordance with your specified formulations, which are brought into contact with each other homogenously in a defined mixing process. Controlled adjustment of the particle-particle contacts enables the resulting overall melting point of the powder system to be lowered. This achieves improvements in the distribution process, which is reflected in the lower quantities required and evenly crosslinked products.
As we package the Combag contents in accordance with your specified weights into hermetically sealed, low-melting PE/EVA bags, here too you no longer have to worry about dust, weighting time, material losses or metering fluctuations.
Continue to: LUVOMAXX® solutions for special crosslinking applications
