Accessment of latex stability

Manufacturer: How do you normally test for the stability of latex as regards its processability?

John Woon: Basically you could do four tests, MST, ZST, ZOV and ZAAV. MST measures the mechanical stability, ZST measures the mechanical stability in the presence of ZnO, ZOV measures the latex viscosity at elevated temperature after addition of ZnO while ZAAV measures the stability in terms of viscosity after adding Zinc in solution i.e. ammoniated Zinc acetate.

The most common test is that of MST but in order to predict more effectively the processability of latex compound in any latex products manufacturing operations, it is best to measure at least one of the other stability tests beside MST.

Storage hardening Natural Rubber

Manufacturer: Most of our NR rubbers in our store tend to harden with time. Some of my friends in the industry refer this to as "storage hardening". Can you explain what this is all about?

JohnWoon: If raw rubber is left standing for some time as would be the case during storage, there is a kind of self cross linking amongst some active groups along the rubber molecular chains. This in turn would increase the viscosity (Mooney) of the rubber, meaning the rubber is harder, hence the phrase, "storage hardening".

Perhaps it would be helpful if we could differentiate here this hardening from that caused by crystallization of the rubber molecules when the rubber is exposed to low temperatures at say, between -40C and -25C. Hardening caused by low temperature could be reversed by just increasing the temperature whereas storage hardening is not reversible.

However, a chemical could be added to the latex before making the dry rubber to prevent the self-cross linking on storage. Hence the rubber would have constant viscosity and is available in the form of SMR CV grade

Natural Rubber replacing Polychloroprene?

Manufacturer: We are making some rubber products using Neoprene (Polychloroprene) as the base polymer. Due to the high cost of this rubber, we're thinking of adding NR to reduce the recipe cost. What is the maximum level of NR we could at?

John Woon: I presume the reason you are using polychloroprene is because we want to achieve good resistance to ozone attack coupled with good resistance to oil for which NR is not a good choice.

You should start by replacing polychloroprene with NR at 8% as a start and check the physical properties of the final cured rubber especially the tensile strength, tear strength and compression set, of course not forgetting the oil resistance and ozone resistance.

You should not exceed 20 - 25% because beyond this level, you would begin to observe a steady drop in oil and ozone resistance.

Poor flocking quality of household gloves

Manufacturer: We make household gloves with reasonable quality which give us quite good ROI for some years. But we believe this could improve if we could overcome the occasional problem of poor flocking of our gloves i.e. the flocks tend to disappear after washing. Any good ideas?

JohnWoon:This question had been raised before. But there is no harm in repeating my comment and recommendation. The basic objective is to have good penetration, anchoring and bonding of the individual strands of the flock onto the latex layer. However over penetration would result in the unwanted total submersion of the flocks. Hence you have to strike a balance between the two extremes.

Please try the following suggestions:

1) Ensure the coagulant for the first latex dip is not too strong otherwise the unused residual coagulant would migrate to the subsequent second latex layer (sometimes referred to as the adhesive layer) leading to premature gelling of this second latex layer. This latex must remain fluid at the time of flocking while in the flocking chamber.

2) For the same reason above, the adhesive latex should not receive unnecessary heating that may result in undue drying.

3) Increase the latex viscosity by adding a suitable thickener at the right level. This would increase the thickness of the adhesive layer.

I am of course assuming that you do not have engineering design problem of the flocking chamber which in the first place should ensure a very even distribution of the flocks throughout the glove surface.

How to minimize biodegradability of Natural Rubber?

Manufacturer:We are producing rubber products based on Natural Rubber. Some of these products must be "submerged" in the soil for many years. Since NR is well known to be biodegradable, we need to prevent this as far as practically possible. For reason we cannot reveal, switching to synthetic polymer is not acceptable. Your assistance would be appreciated.

JohnWoon:Unvulcanized NR would biodegrade faster than vulcanized NR and among the vulcanized NR products, those with better protection against oxidation would be more resistant to biodegradation.

I suggest you try the following ideas:

1) Use an EV system for your vulcanization recipe. EV stands for 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, offers good resistance to oxidation especially at elevated temperatures.

Research done indicates that while giving low stress relaxation, such formulation also enhance the resistance to biodegradation.

2) As normal organic waxes would trigger and increase biodradability, this material must be avoided or replaced with chlorinated wax.

3) Use a strong anti-degradation system. One possible combination is p-phenylene diamine antioxidant with chlorinated wax. Phenolic type of antioxidants such as butylated phenols, would not be strong enough.

Effect of bacteria on NR latex

Latex Producer: What damage can bacteria do to NR latex and how to prevent this?

John Woon: The main problem is the by products of the bacterial attack on the carbohydrates, amino acids, inositol (particularly quebrachitol), proteins etc present naturally in the latex. These are the undesirable acids, namely, acetic, formic and propionic acids, the so-called VFA (Volatile Fatty Acids). They reduce latex stability, increase viscosity and coagulum formation.

Contamination of latex with bacteria starts at the time the rubber tree is tapped. Although it is difficult to control and prevent bacteria infection at this stage, steps can be taken to ensure that there is no further undue infection by adhering to stringent cleanliness control of all equipments in all subsequent processing, namely containers, tanks, tankers, centrifuges, storage tanks etc. Also, latex once collected must be processed as soon as possible without undue delay. Ammoniation must be carried out soonest possible without delay.

Bacteria trapped in coagulum are somewhat protected against bactericide and would emerge and cause havoc later when given a chance. Hence all coagulum and dried skins must be removed and discarded.

Creaming of latex

Manufacturer: We are adhesive manufacturer and started to buy NR latex in 200 kg steel drums about a year ago. Whenever we want to use the latex, we always find a thick layer of latex at the top of the drums. This has given us some problems. How can we prevent this?

JohnWoon: Before I start advising you how to overcome this problem, perhaps it'd better if I give some basics about the latex colloidal properties. Latex is basically a dispersion of the rubber particles in the serum (i.e. the aqueous phase of the latex). The rubber particles, being lighter than the serum, would tend to move upwards and rise to the top of the latex especially when left alone without agitation. This would result in a thick layer with very high solid content that can be as high as 70% and more while the normal solid content is about 60%.

This is what we call the "creaming process" which occurs faster at higher temperatures (25C to 30C). The rate of creaming is lower at lower temperature (10C to 25C). I suggest you agitate the latex by stirring it with a collapsible stirrer or by rolling the drums. If possible, empty some drums of latex into a tank and stir the latex in this tank periodically, say every 2 to 3 days for about half an hour each time.

Do not discard the "thickened" or "creamed" latex unless it has dried to solid state. This latex is still reversible when remix in normal latex.

Advantages of Prevulcanised latex

Manufacturer: You seem to be an advocate of Prevulanised latex. Why?

John Woon: From the point of view of a latex products manufacturer, the use of prevulcanised latex simplifies a great deal the whole manufacturing process. The seemingly exaggerated and oft-repeated cliche, "Just add water to the PV and start running the machine" actually holds some truth.

Depending on what products are being made, the prevulcanised latex could be used as it is (as in the case of toy balloons after the addition of pigments) or after some dilution with water to achieve a final latex solid content of as low as 30% (as in the case of examination gloves). What one needs to do next is to mix for about 30 minutes and allow enough time, usually 16 to 24 hours for deaeration (i.e. for air bubbles to escape) before the latex is ready for dipping, casting, extrusion, spraying, painting, coating etc.

No maturation period is required as in the case of post-vulcanisable latex compound where a "maturation" stage is almost always a prerequisite for making reasonably good quality latex products.

"Maturation" is a stage when sufficient time must be allowed for both the naturally occurring and added surfactants and fatty acid soaps to reach an equilibrium. Also, a controlled degree of vulcanization must take place during this stage before the latex compound is ready to be used. To use the latex compound too early or too late would result in under-curing and over-curing respectively.

Generally speaking, unlike a prevulcanised latex, post-vulcanisable latex compound would have a marching curve immediately after compounding in terms of the degree of vulcanization. (Figures 5 and 6)

Hence, in the case of post-vulcanisable latex compound, it is more difficult to prevent situations of over-curing when cracking and tearing of, for instance, gloves and condoms are frequently encountered. This is attributed to the fact that the tensile strength reaches a peak before reclining as the cross-link density increases. In short, post-vulcanisable latex compound has short shelf-life of usually from 2 days to 2 to 3 weeks depending on the curative formulation. On the other hand prevulcanised latex generally has a very much longer shelf-life of 6 to 9 months.

Therefore less stringent process controls are required for prevulcanised latex. Also, very often, one finds the viscosity of such compound increasing with time, unlike prevulcanised latex. This is basically a result of zinc ammine thickening. This involves the dissolution of zinc oxide by ammonia in the presence of ammonium salts releasing zinc ammine complex ions which in turn would react with the stabilisers on the latex particles namely the fatty acid soaps and proteins forming insoluble zinc soaps and proteinates. The end result is the loss of latex stability accompanied with increasing viscosity.

It goes without saying that the vulcanization stage is not required for prevulcnaised latex although, in practice, an oven is still required to accelerate the drying.

Another important advantage is the low residual chemicals, particularly the accelerators. This results in a "cleaner" latex compound with low toxicity level which is of utmost importance for the manufacturing of medical devices such as gloves, baby teats, condoms, catheters and medical tubings. Testing of these articles have been carried out by medical device manufacturers taking into account their destination and service conditions. This includes chemical analysis of extracts, skin irritation, skin sensitization, muscle implantation, pyrogeneity, cell cytotoxicity etc.

Experience shows that latex medical articles made from prevulcanised latex are in most cases biologically less active than those made from typical post-vulcanisable latex compounds. Biological activity is usually caused by ingredients that have not fully reacted in the process of vulcanization. These could migrate into human bodily fluid such as fluid from mucous membranes, blood, saliva and other physiological fluids.

Study has been done to compare the efficiency of leaching of films made from prevulcanised latex and post-vulcanisable latex compound particularly during the so-called post-leaching or dry leaching process. The findings indicate that a prevulcanised latex film offers more efficient leaching when post-leaching is carried out.

However, there are some disadvantages of prevulcanised latex. These are the inherently darker colour and the higher tackiness.

Understanding "Vulcanisation of latex"

Manufacturer:I understand you have answered this question of mine some time back. But hope you could answer again. My question: Is vulcanization of Natural Rubber latex similar to that of the dry rubber? What is the mechanism involved?

John JohnWoon: The possibility of vulcanizing the rubber molecules within the dispersed rubber particles in the Natural Rubber (NR) latex was first investigated by Philip Schidrowitz in the period 1914 – 1918. His idea was to use prevulcanised NR latex for making latex foam products in order to eliminate the vulcanization stage for economical reason.

Most of the subsequent studies by Schidrowitz and other researchers found that prevulcanised latex was generally not suitable for foam products due to the inherently poor gel strength.

(However, with some modification, a "hybrid" system based on prevulcanised latex could now be used successfully for making foam products such as mattresses and pillows).

While the chemistry of dry rubber vulcanization is complex, the mechanism of vulcanisation of rubber particles in latex is even more so and remains a wonder even today, almost 90 years after the advent of the discovery of prevulcanisation of latex. Unlike dry rubber, where the curatives could be intimately mixed and dispersed within the rubber matrix itself, similar curatives remain dispersed and suspended separately in the latex along with the rubber particles at least immediately after compounding and mixing. At best, the curatives could only collide with the rubber particles.

Many researchers had put forward their different schools of thoughts over the past decades. While some of these were contradictory with one another, others raised more questions

It seems likely that the first important step in the vulcanization of latex is the formation of sulphur-accelerator species in the serum which then becomes soluble in the presence of the hydrophilic non-rubbers. It is possible that the resultant active sulphurating agent attains some degree of surface active property. Hence a logical mode of transfer would be its absorption onto the rubber particle surface from the serum or the aqueous phase.

Once on the rubber particle surface, the sulphurating agent would loose some of its hydrophilic moieties resulting in an increase in the hydrophobicity enabling itself to migrate into the hydrophobic interior of the rubber particle to trigger the initial formation of polysulphidic cross-links followed by cross-link shortening with recycling of the sulphur into additional cross-linking, modification of polyisoprene chains etc.

Prevulcanised latex is therefore a compounded latex in which the molecules of the rubber particles are chemically cross-linked (i.e. vulcanised). However there is no change in rubber particle size, shape and particle size distribution, the latex still retaining its original fluidity and colloidal property.

Formulation could be designed to give different types and degree of cross-linking resulting in a varied range of prevulcanised latex for making products having different modulus with M700 (Modulus at 700% elongation) ranging from 9.0 MPa to 19.0 MPa, high heat resistance, low nitrosamine and nitrosatables, low copper-staining, sterilization resistance etc. All these could be made available both in high (0.6%) ammonia or low (0.3%) ammonia version.