Latex "strike-through" problem

Manufacturer: We make several types of latex coating compounds, one of which is a textile product which cures on contact with a pre treated garment however the reaction isn't quick enough and the coatings tend to sink into the garment too fast before the droplet has had time to cure on the surface, currently our pre treatment contains calcium nitrate.

We have tried increasing the levels of calcium nitrate into the pre-treatment formulation, but this causes problems with the bleaching effect that it has on the textile dyes in the substrate. Also in terms of viscosity increase, we are stuck with the levels that the coating will allow (extremely low).

There is a surfactant in the coating in the form of sodium dodecyl sulphate. Would this be likely to cause a problem?

Can you suggest any reactants that cure a polymer with quicker reaction times like almost instant as soon as the latex lands on the treated garment?

JohnWoon: What you're having is what we call "striking through" i.e. unwanted and excessive penetration of the latex or other liquid into the matrix of the textile.

Please ensure that there is minimal or zero level of non-ionic surfactants in your coating formulation as this would retard the gelling or setting of the coating when coming into contact with calcium nitrate.

Check and see if you could reduce the level of SLS (sodium dodecyl sulphate) but not to the extent of the latex stability being compromised. The effect of such reduction is also to control or reduce the wetting property of your ink so as to minimize "strike through".

Please also look into the possibility of replacing calcium nitrate with ferric chloride or zinc nitrate/chloride. Deliberate but controlled destabilizing of the latex could also help.

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How important is the particle size of chemicals for latex products?

Manufacturer:
We are manufacturing both Nitrile and natural rubber latex gloves. Recently we’ve heard of chemical dispersion of sub-micron nano size being offered in the market. Is it important to use curative dispersion of nano size to ensure good physical properties of our gloves?

JohnWoon: Earlier work done by MRPRA in England suggests that the answer is "NO". This is because of the possibility of the curatives, namely sulphur, ZnO and accelerators forming sulphur-accelerator species in the serum of latex and becomes soluble very quickly in the presence of hydrophilic non-rubbers. Curatives of average particle size ranging from 1 to 5 microns have been and are being used without problems.

Also, ZnO is known to dissolve almost immediately when added to Nitrile latex due to its reaction with the carboxylic acids. Hence the question of the settling of the high-specific gravity ZnO and hence poor cross-linking does not arise.

Having said that, chemical dispersions with very fine particle size would be advantageous in reducing the degree of sedimentation of the particles in the dispersions on storage. This would make the job of pre-mixing the dispersions just before adding them to the latex easier. Also, if fillers are used, nano-sized versions are preferred.

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Water sensivity of latex products

Manufacturer: We are using potassium soap (fatty acid based) in our latex. Owing to production constraint we cannot implement the leaching process during or after production although our finished latex products are to have good resistance to water absorption. Please advise how best we could improve the resistance to water absorption without leaching. We do not use coagulant for our process.

JohnWoon: I’m afraid I have no easy solution for you if you do not intend to do leaching. At best my advice could only help you to have a partial improvement.

Try the approach of what I call the “fugitive soap technique”. Select soaps/surfactants that would revert to the corresponding fatty acids on drying. These are the ammonium and amine soaps. I must stress that you use this remedial action only as an interim step to solve your problem in the short term. I strongly advise that you set a long term objective to introduce a leaching stage in your process.

Good luck!

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Molding of thick rubber products

Manufacturer: I know your area of expertise is in latex technology but we have a problem with dry rubber moulding. I was told that you might also be able to help us. We are moulding articles which are quite thick and are experiencing reversion of the rubber because of high curing temperature and long cure time. Please give us some guidance.

JohnWoon: No problem! I shall attempt to handle dry rubber problems as well and hope my suggestion would be useful for you.

"Reversion" or over-curing is always a problem with sulphur vulcanization because once the optimized required x-linking density is reached, the reaction does not stop if the rubber is exposed to elevated temperatures. This problem is always evident in the case of thicker articles because rubber is a poor conductor of heat.

In the attempt to ensure the interior of the article receive adequate vulcanization, the exterior or areas near to it could very often be over-cured and you end up with a product that is under performing. I suggest you try the following remedial actions:

1) Preheat the rubber just before moulding but not to the stage of "scorching".
2) Scale down your existing cure temperature at the initial stage of the cure. This is to allow the rubber to get heated up slowly through out its mass without overheating the exterior.
3) Gradually increasing the temperature.
4) Increase level of ZnO if your formulation could take it because ZnO renders better thermal conductivity to the rubber.
5) Use EV system in your cure formulation. If you don’t understand what "EV" is, it is "Efficient Vulcanisation" based on sulphur-donor in the absence of sulphur or at very low level of sulphur. The resultant mono-sulphidic cross-links, as opposed to polysulphidic ones from the conventional cure system, offer better resistance to oxidation especially at elevated temperatures.

I’m expecting a drink from you after your trials. Good luck!

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BASF's increased production of award winning non-phthalate plasticizer

According to a report, BASF's increased production capacity of its specialty plasticizer Hexamoll® DINCHwill is, if not already, ready for commercial production now.

The non-phthalate plasticizer (1,2-cyclohexane dicarboxylic acid, di-isononyl ester) was especially developed for use in sensitive applications such as toys. It also states that the plasticizer is compliant with the new EU regulation on the restriction of phthalates in toys and children articles and is already widely used
by the toy industry in Europe and Asia. In addition to medical applications such as gastric feeding tubes, BASF reports that it is also receiving enquiries about Hexamoll Dinch from manufacturers of other flexible PVC products, like garden hoses, or for sensitive non-PVC applications like children’s modelling material.

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