Pigments and Latex Stability

Manufacturer: We just started using a few batches of pigments from a new supplier but encountered sleepless nights due to latex stability problems. Please give us a simple guide.

John Woon (Senior Latex Consultant): Some pigments increase the stability of latex, but others reduce it. You can confirm this by checking the viscosity and coagulum level of the latex compound both before and after the addition of the pigments. 

Some pigments contain “glycol compounds” that help improving the pigment dispersion stability but could destabilize the latex. Glycol is commonly used in making some pigment dispersions or pastes.

 

How Does Temperature Affect the Colloidal Stability of Latex?

Manufacturer: Lately we have been facing difficulties in controlling latex temperature due to high summer temperature within the factory production area. At the same time, we observed a high rate of rejection due to latex destabilization (coagulum formation). How does heat affect latex stability?

John Woon (Senior Latex Consultant): The kinetic energy of the rubber particles is increased when the latex temperature increases, for instance, when the ambient temperature of the factory is elevated during summer, the rubber particles tend to move at a much faster rate. This results in an increase in both the frequency and violence of the collision of rubber particles with each other. 

Furthermore, the elevated temperature of the latex will result in a drop in the pH due to the loss of ammonia, leading to latex destabilisation. The end result is coalescing and coagulation.

Natural rubber latex is therefore best stored at temperatures ranging from 15°C to 30°C. A water chiller should be used, especially in the latex dip tank.

Poor Tensile Strength of Gloves

Manufacturer: We are facing the persistent problem of poor tensile strength in our gloves despite having improved our latex compound formulations. Can you help?

John Woon (Senior Latex Consultant): In the manufacturing of latex-dipped products including gloves, the physical properties (including tensile strength) of the finished products, do not merely depend on the compound formulations.

The dipping process requires perfect wetting properties of both the coagulant and latex, along with clean formers without micro-flaws. You should also look into the effect of occlusion of foreign particles, air bubbles, pinholes, etc.

"Wetting" is a big subject that must be well understood before venturing into making latex-dipped products. If you want, I can prepare a separate report with more technical details.

Modulus of Latex Products

Manufacturer: We are manufacturers of latex products. One of our customers requested that our products be harder and softer but without changing our curative formulations. How shall we proceed?

John Woon (Senior Latex Consultant): Technically you are talking about the "modulus" of the rubber which depends on the degree of cross-linking and cure state. The higher the modulus the harder the rubber. And the reverse is true.

Your customer asked for higher and lower modulus probably because they are concerned about the processability of the latex compounds if the curatives are changed.

I suggest you try adding MMA-grafted NR latex (or some synthetic latices) for higher modulus while adding plasticizer (e.g. mineral oil) for lower modulus.

The Need for Maturation of NR Latex Compounds

Manufacturer: We are experiencing poor physical properties, especially the tensile strength, of our NR latex products made from the casting and extrusion processes. This is despite the fact that our latex compound formulation has been optimised in the lab with the right cure temperature and cure time, with the help of a consultant. Your input on how we could overcome this problem would be highly appreciated.

John Woon (Senior Latex Consultant: You might be having "precure" problem. The latex compound should be sufficiently matured to allow for some degree of cross-linking (i.e., prevulcanisation) to take place before commencing with further processing. 

The required degree of "prevulcanisation" could be determined with the Chloroform Coagulation or Toluene Swell test. An insufficiently matured latex compound could lead to poor tensile strength even if you increase the cure temperature and/or cure time.

Wetting Property in Latex Dippingg Technology

Manufacturer: We are a bit confused when we were told by some experts from one of our universities that dipping technology using latex can only be successful if the wetting of the coagulant and latex is optimized, with the right surface tension of the liquid and surface free energy of the substrates. Can you please explain is simpler terms for our understanding?

John Woon (Senior Latex Consultant): The university's experts were right. The success of the dipping process depends a lot on the principles of “wetting”, i.e., how well the coagulant wets the former surface to achieve an even coating of the coagulant and hence subsequently, an even coating of latex on the former.

For good wetting property of a liquid over the surface of a solid, the liquid must have a low surface tension while the solid must have a high surface free energy (or surface energy). 

The more “polar” the surface is, the higher is the surface energy. Surface energy of solid surface must always be higher than the surface tension of the liquid, in order to achieve good wetting. In the manufacturing of latex gloves, this simply means that the coagulant must have low enough surface tension of around 30 to 32 Dynes/cm while the formers must have clean and uncontaminated surface.

Former materials with high free surface energy (e.g., 200 to 1000 Dyens/cm) include metal (aluminium, steel), glass and ceramic. Plastic formers have the lowest surface energy (usually below 60 Dynes/cm) unless it is modified by some treatments.

 

Latex Colloidal Stability and Pigment Choice

Manufacturer: We are a manufacturer of latex-dipped products in different colours. We find some pigments have different effects on our latex compounds in terms of latex stability. We would like to know why.

John Woon (Senior Latex Consultant: Some pigments increase the colloidal stability of latex, while others reduce it. You can easily confirm this by checking the viscosity and coagulum level of the latex compound both before and after the addition of the pigments. Pigments with “glycol compounds” are particularly bad for latex stability. Therefore, the remedial action is obvious.

If you have knowledge of the basic principles of latex stability, you are better equipped to overcome the stability problem faced with some pigments. If you need further input on this subject, please let me know.

 

Manufacturer: Could you please suggest me replacement for LATZ preservation for natural rubber latex?

John Woon (Senior Latex Consultant): LATZ can be replaced with HA (High-ammonia concentrate).

Latex Gloves with Thin Spots

Manufacturer: We are experiencing a lot of problems with thin spots in our latex gloves. We have taken many actions but the problem remains unresolved. Please help us.

John Woon (Senior Latex Consultant): This very common problem is caused by many factors. It is beyond the purview of this forum to discuss in detail all the possible causes. But I would give you some practical tips to help you in your further investigation.

Among the possible causes are the followings:

1) Dirty formers

2) Insufficient prevulcanisation

3) Poor wetting of coagulant

4) Former temperature too low on dipping into latex

5) Fluctuating coagulant temperature above and below the cloud point

6) etc.

If you require a more comprehensive report with a more detailed discussion, please let me know.

Manufacturer: We have been experiencing bad quality of our latex many times, in terms of latex stability, especially MST (Mechanical Stability Test). We are forced to add a higher level of soap. How can we overcome this problem for good without having to do "firefighting" all the time?

John Woon (Senior Latex Consultant): First of all, where is the source of the latex concentrate? Who is the latex concentrate producer (producers)? Have you done an on-site audit of their premises? How good is their standard of hygiene and quality controls? Do you have your own latex specification or do you depend on their proposed specification?

You need to answer all these questions first before devising methods or remedial actions for the latex stability problem you are now encountering in your factory.

Detailed advice on this problem including the full parameters of the latex specification is beyond the purview of this platform but I want to at least give you a good hint. Check the VFA Number which should not be more than 0.03. This important parameter indicates how good or bad the producer has been doing their job.

It is of no use to accept bad quality latex and then try to make it good at your end for your process, by adding more soap which could lead to other problems such as foaming, webbing, poor latex gelling, etc.

If you want a more comprehensive and detailed report on this, please contact me separately.

Freeze-Thaw Stabilitiy of Rubber Latices

Manufacturer: We are manufacturer of rubber products based on natural rubber latex (i.e., HA and LATZ from Thailand). Our problems are due to latex instability during winters caused by the freezing temperature. What can we do to solve this problem?

John Woon (Senior Latex Consultant): What you are experiencing is the effect of freezing and thawing of the latex due to the lack of proper temperature control during transporting and storage in winters. The symptoms are usually an increase in latex viscosity, drop in MST and the formation of coagulum. The mechanism of the destabilization involves the aggregation and coalescence of rubber particles forced by the ice crystals formed in the serum of the latex.

The immediate remedial action you could take is to dilute the latex slightly with alkali water followed by filtration and addition of stabilizers such as ammonium laurate.

For a long-term solution, you should request your suppliers of latex to offer you a freeze-thaw stable LA or LATZ which is usually produced with the addition of a freeze-thaw stabilizer such as sodium salicylate and ammonium laurate.

Leaching to Reduce Toxicity Level

Manufacturer: We are making medical products using NR latex compounds, which require a very stringent control of toxicity level of our products. We are attempting  improve our leaching process. Please give us some guidance.

John Woon (Senior Latex Consultant): I had answer many questions on leaching before. Please refer to my previous answer as follows:

Leaching is a big subject which entails at least 7 steps of mechanism. I can prepare a detailed report for you if required. In the mean time, here is my brief suggestion:

1) Higher leach water temperature is to be used to ensure maximum solubility of all the hydrophilic materials. But cold water leaching is to be included if non-ionic surfactants are also used.

2) Frequent replenishment with clean water is advisable.

3) Turbulence in the leaching tank would help.

4) Leach for as long a period as practically possible.

5) Adopt both wet and dry leaching.

6) Check copper (ion) level in the leach water.

If you are using a post-vulcanisable latex compound, I suggest you evaluate prevulcanised latex which has a comparatively low level of toxicity.

 

Prevulcanised Latex vs Post-Vulcanisable Latex Compounds in Leaching and Drying

Manufacturer: Due to the length of our dipping lines, our oven length is too short for sufficient drying and leaching. Our local university advised that we should look into using prevulcanised latex. But we do not understand why? Would appreciate your simple explanation. Thanks.

John Woon (Senior Latex Consultant): Here's my simple explanation:

This has a lot to do with the particle-particle integration of the rubber particles during drying. Unlike the post-vulcanisable latex compounds, a prevulcanised latex (PV) gives rubber films that are more porous due to lower extent of particle-particle integration (i.e. inter-entanglement and inter-diffusion of the rubber molecules of different particles).

As a result, latex films from PVs, when compared with those from post-vulcanisable latex compounds, will dry faster and are easier to leach.

Poor Wetting Problem of Plastic Formers for Toy Balloon Manufacturing

Manufacturer: We are toy balloon manufacturer, using formers made from plastics. Occasionally, we are hit by high rejection rate and we suspect poor wetting of the plastic formers as the major cause because when we use ceramic formers, we do not have similar problems. We understand that some manufacturers practice some kind of treatment on the formers before use. What is this treatment?

John Woon (Senior Latex Consultant): Yes, such poor wetting problem is to be expected because the plastic formers, especially polyolefin plastics, have comparatively low surface free energy compared to ceramic formers.

You could try roughening the surface of the plastic formers with sand paper followed by etching with acids such as sulphuric acid and chromic acid. Other treatments include corona discharge, flame and plasma. But all these treatments should have been done by the plastic former manufacturers.

What to do with old over-matured latex compounds?

Manufacturer: Would you please suggest some inputs on how to utilize old compounded latex collected from the dipping tank ?

During plant shutdown, compounded latex from the dipping tank is collected and stored but later becomes over-matured.When resuming dipping operations, some quantities of over-matured latex are mixed with fresh compound but the defect rate increases. Can you please suggest some ideas based on your experience in this regard?

What are the properties to be tested for the over-matured latex before using? Is it possible to use the over-matured latex for some other purpose?

John Woon (Senior Latex Consultant): The use of old and over matured compounded latex for dipping is not recommended because it tends to give cracking during the dipping process due to weak gel strength and the final products will also exhibit poor tensile properties with low tensile strength and elongation at break. This is due to the poor rubber particle-to-particle integration during drying.

You can mix a low percentage of the over-cured latex with fresh latex compounds, treating it as a filler.

The quality of the over-cured latex could be accessed by determining the physical properties of the cast films. These include tensile strength, modulus, elongation at break and tear strength. Based on these results, you could then decide on the ratio of the blend to be used.

Needless to say, the film properties of the blend must also be tested to ensure that the final products are still acceptable as per your own quality assurance requirement.

Perhaps you could look into using the over-matured latex compounds for other applications such as carpet backings and impregnation of textile such as non-woven fabrics. However, some lab works should be carried out to access the suitability of this approach.

 

Uneven Coating of Latex on Formers

Manufacturer: We are glove manufacturers. Occasionally we are experiencing uneven latex pick-up by the formers resulting in uneven coating of latex on the formers.We accept the usual differences in glove thickness from fingers to the cuff but the uneven latex coating is random and a real pain!

John Woon (Senior Latex Consultant): Please look into the followings:

1) Check the cleaning process of the formers to ensure that cleaning with both acid and alkali is effective.

2) The formers should be properly rinsed after washing and dried before entering the coagulant tank. 

3) Ensure the coagulant has an efficient wetting agent.

4) The formers should be hot before the coagulant dip and the heating of the formers should be even.

5) Check the surface tension of the latex to ensure that it is low enough to give good wetting on the formers.

For other related details, please refer to me separate report.

Halogenation of Latex Dipped Products

Manufacturer: We are in the process of experimenting with halogenation treatment of our latex dipped products to remove the surface tack, using chlorine gas (in water), sodium hypochlorite (with acid) and bromine water. Which one do you recommend?

John Woon (Senior Latex Consultant): The effectiveness of tack removing of chlorine is quite similar to that of bromine. Both are better than sodium hypochlorite in this regard. Among the three, I would choose chlorine because it gives the east discolouration to the rubber.

Please also refer to my report on "Chlorination of Rubber Products" for other details.

Colloidal Stability of Compounded Latex

Manufacturer: We have an experience of coagulation of compounded over matured latex kept in barrels. Normally, during plant shut down compounded latex left in the dipping tank is collected in barrels and kept as over-matured latex.

How did the coagulation of compounded happen? Is it because of poor preservation ? or due to some other molecular change due to chemical reaction of compounding ingredients? How is the bacterial attack possible with the presence of compounding chemicals?

John Woon (Senior Latex Consultant): Compounded latex does not store well unless it is a properly prepared prevulcanised latex.

The presence of Zn ions from ZnO and zinc salt of dithiocarbamates could destablise the latex. You could control but not eliminate it totally. Of course, if the original latex concentrate has not been properly preserved and produced, it could make the situation worse!

 

Make sure the parameters of latex specification are suitable for your purpose, especially the VFA Number and MST.

Please adjust the pH of the latex to about 10.00 and add extra soap such as potassium laurate followed by a slow and gentle filtering process.

 

Problems in Dipping with Polychloroprene Latex

Manufacturer: We are manufacturers of latex dipped products using both NR latex and polychloroprene latex. While we are happy with our NR latex-based products, we are encountering some problems with our polychloroprene latex compounds. We are not able to overcome the problems of surface roughness and horizontal lines. Would appreciate your input on how we should approach these problems.

John Woon (Senior Latex Consultant): Dipping with polychloroprene (CR) latex requires special attention to the rate of dipping and withdrawal of formers in the latex dip tank. Vibration of the formers must be kept to the minimum if this could not be totally eliminated.

Also, the wetting properties of  CR latex should be improved by judicious addition of wetting agents (i.e. surfactants). Good examples of such surfactant are sodium salt of sulphated methyl oleate and sodium alkyl sulphate.

For more details, you have to refer to my report on "Dipping with Polychloroprene Latex".

Multiple Latex Dipping and Delamination Problem

Manufacturer: In one of our dipping processes, we are doing double latex dipping with the first dip in the latex followed by coagulant and a second latex dip. We are observing some technical problems of the final products. What is you comment of such dipping process?

John Woon (Senior Latex Consultant): The advantage of dipping the former into latex as the first step is that it could overcome slippage problem of the latex gel or film from the formers especially with wet coagulant (such as acetic acid), smooth/glazed formers and to a certain extent the shape of the formers.

Since it involves two latex dipping, the first latex and the second latex layers might not integrate sufficiently well due to the presence of a layer of the coagulant between them. This could therefore lead to delamination. 

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Webbing Problem During Latex Dipping

Manufacturer: We are a radiation protection glove manufacturer from Mumbai, India. Nowadays we are suffering problems with webbing issues. We tried different types of anti webbing agent like Rhodoline 642 NI, Freesil 622 etc. But we can’t get proper solution. When we add more deformer, webbing problem gets solved and in that same time gloves will shows fish eye. Please suggest any solution.

John Woon (Senior Latex Consultant): Certain batches of latex do give persistent webbing problem. Instead of adding the anti-webbing agent during the latex compounding and mixing, you should add it drop-wise on the surface of latex in the vicinity of the former withdrawal regions in the latex dip tank. Reducing the latex TSC and slowing down the former withdrawal speed could also help.

Monomer ratios in SBR latex

Manufacturer: What are the usual monomer ratios in SBR latex which we are using for our carpet backing compounds?

John Woon (Senior Latex Consultant): The usual level of commercially available carboxylated SBR is in the range of 45% to 60%. Lower level of styrene could have poor tensile strength while too high a level could lead to low elongation and tackiness while the film formation requires higher temperatures since the Tg is increased. You should choose a range of the monomer ratios (Styrene/Butadiene) that give you good film formation and tuft-bond strength of the carpet while achieving good modulus and tensile strength.

"External Plasticizing" and "Internal Plasticizing"

Manufacturer: We are manufacturer of water-based adhesives using polyvinyl acetate emulsion (latex). Please explain the meanings of "External Plasticizing" and "Internal Plasticizing".

John Woon (Senior Latex Consultant): These terms are used quite frequently in plastic technology, where rigid plastics are rendered soft by adding a plasticizer (softener). A good example is the rigid PVC used of making rigid PVC pipes. The same PVC material can be softened by adding plasticizers for the manufacturing of soft and flexible hoses. Such plasticizing effect is known as "External Plasticizing". However, the addition of "external" plasticizer could lead to "sweating", where the plasticizer migrates and bloom on the surface of the plastic products. As a result, the products concerned become more rigid again.

Another more permanent method of making the polymer/plastic softer is through "co-polymerization". In one case, rigid Polystyrene can be made softer by co-polymerization with another monomer, butadiene. This is known as "Internal Plasticizing".

Same principle also applies to polyvinyl acetate homopolymer and co-polymers.

Curing Latex-Based Products with Boiling Water

Manufacturer: Hi Mr. John Woon. We have learnt a lot from your postings over the years. I'm sure you could help us again here. My question: Besides steam and oven with different heating systems, what else could we use for the curing of our latex based products.

John Woon (Senior Latex Consultant): Have you tried boiling water? If not, I suggest you conduct some experiments using boiling water to cure your products with different types and levels of zinc dialkyldithiocarbamates as the accelerators. I have used this method myself and there are a number of advantages compared to ovens as shown below:

1) The plateau of the curing curve is longer and therefor there are lesser chances for "reversion".

2) There is better interaction between the curatives and the rubber because of better heat transfer.

3) The absence of air (and hence oxygen), deterioration of the rubber is lesser.

4) The removal of hydrophilic residual materials, residual accelerators and by-products of vulcanisation is more efficient.

5) The final products are therefore cleaner.