Shrinkage and Carbon Black 1

[Elastomatt]

Hello Again!

Just a quick question today....can changes in carbon black (supply rather than grade) change the shrinkage of the material?
I have been led to believe that the polymer used has more impact on the shrinkage.

Basically I am trying to find out what affects changing different components of a compound would have on the shrinkage (polymer, carbon lack, oil and cure system).

Thanks

Elastomatt

 

[niki29]

The more carbon black is in compound the lower shrinage is.

 

[WCRPLtd]

Changes in the supply of carbon black should have very little effect on shrinkage so long as the grade is the same and the suppliers are reputable! By far the more important factors are a) the viscosity of the gum polymer and b) the level of filler. High viscosity polymers can handle a higher degree of fillers, a highly extended compound should be less susceptible to shrinkage. The make-up of the fillers and especially the particle size (surface interaction) can have a bearing but are of less significance.

http://www.white-cross-rubber-products.co.uk

 

[GrahamBennett]

Are we talking about shrinkage after cure or after extrusion or calendering?

Please bear with me on what I write next, as it talks about nerve rather than shrinkage (but they are very similar):

Nerve is the elastic recovery that compounds exhibit after deformation. While a pronounced elastic response is necessary in the application, if it excessive in the rubber’s uncured state it can cause processing problems. Therefore, it must be reduced during mixing to improve processing, dimensional stability, die swell and calender shrinkage.

Methods of controlling nerve:

1. Mastication reduces nerve.
2. Increasing filler loadings will reduce nerve.
3. Using high structure in place of normal structure carbon blacks will reduce nerve.
4. The addition of factice, mineral rubber, or reclaim can be useful but one can expect some loss of physical properties. For example, use WB grades of CR although T-types are available that do not show nerve in later processing stages.
5. Calendering at higher temperatures will also reduce nerve but can cause the compound to stick to the rolls.

Control of shrinkage during extrusion or calendering is down to several things including take off belt speed. If the take off speed is greater than the speed the rubber is leaving the extruder die or calender bowl then you can expect varying levels of shrinkage before curing. If you match the speeds more closely (ideally take off speed is 1-2% faster) then stretch is minimised; thus making shrinkage much less of a problem.

 

[Elastomatt]

Thanks to all, I am mainly concerned with moulding and extrusion shrinkages.

Given me lots to think about (and look up!) ;-)