Wetting Property in Latex Dipping

Manufacturer: We want to know more about wetting in latex dipping. We are a manufacturer of latex gloves and often have problems with the wetting of the formers.

John Woon (Senior Latex Consultant): Wetting property is a complex subject that cannot be fully explained on this platform.

However, I will give a brief explanation. For excellent and ideal wetting property of a liquid (e.g, coagulant) over the surface of a solid (e.g., glove former), the liquid must have a very low surface tension, while the solid must have a high surface free energy (or surface energy).

Therefore, in latex dipping, the surface tension of the coagulant mix should be adjusted to 30 to 32 Dynes/cm (water having a surface tension of 72 Dynes/cm).

To maintain high surface energy, the formers must be thoroughly cleaned after each cycle of dipping.

If you want a more comprehensive report on wetting, please let me know.

Controlling of Cure State of Latex Compounds

Manufacturer: We are facing problems of controlling the cure state of our latex compounds, resulting in our products being either tacky or having poor tensile strength.

John Woon (Senior Latex Consultant): The latex compound should be given sufficient time to mature at room temperature before commencing your dipping process.

The Chloroform Stage should be between 2 and 3, while the Toluene Swelling should be between 95% and 100%. If the latex compound is not sufficiently matured when used in the dipping process, under-cure of your products will ensue, and this will lead to a tackiness problem.

As a result, even when the curing temperature is increased and/or the cure time is increased,  the tackiness problem could still persist.

On the other hand, if the latex compound is over-cured (i.e., Chloroform stage 4 and toluene swelling more than 150%), the products would have poor physical properties, for example, poor tear strength and tensile strength, while the elongation at break is low.

 

Surface Tension and Surface Energy in Latex Dipping Process

Manufacturer: We are very confused by the terms surface tension and surface energy when dealing with wetting properties in our latex glove dipping process. Hope you could give us some simple explanation.

John Woon (Senior Latex Consultant): This is quite a complex subject, but I'd attempt to make it as simple as possible for you.

"Surface tension" and "surface energy" are the same, but usually one uses "surface tension” when referring to liquids (e.g., coagulants and latex) and “surface energy” for solids (e.g., formers). Also, “surface energy” is sometimes referred to as “surface free energy”.

In the manufacturing of latex gloves, this simply means that the coagulant must have a low enough surface tension of around 30 to 32 Dynes/cm, while the formers must have a clean and uncontaminated surface to have a high surface energy. Generally, a ceramic or glass former is superior to a plastic former.

The surface tension of the water is too high at 72 Dynes/cm, which makes it a very poor wetting liquid, hence the need to add a surfactant (i.e., wetting agent) to the water when preparing the coagulant mix.

Similar consideration should also apply to the latex compounds.

KOH Number of Natural Rubber Latex Concentrates

Manufacturer: Our consultant advised that we should not pay too much attention to the KOH number of our incoming latex concentrates. What is your comment?

John Woon (Senior Latex Consultant): I disagree with your consultant. 

KOH Number is defined as the number of grams of potassium hydroxide equivalent to the acid radicals in the latex combined with ammonia containing 100 gm of total solids.

There are different types of such acid radicals in the latex, which are measured with the KOH Number. Some are good while others are bad. The good ones are the fatty acid soap and protein ions, which are latex stabilizers. The bad ones that can destabilise latex are amino acids (by-products of the breakdown of proteins) and the volatile fatty acids (VFA). Other bad ones include phosphates, carbonates, citrates, and sulphates.

Therefore, this parameter must be accessed together with other parameters to access the quality of the latex.

If you are getting the latex from the same source or plantation, the KOH Number could indicate the age of the latex. The higher the number, the older the latex, and hence, the lower the latex stability.

I suggest you liaise with your suppliers and set the specification of KOH Number at 0.6 (maximum).

Effects of Pigments on Latex Stability

Manufacturer: We are a toy balloon manufacturer. Very often, we encounter a latex destabilisation problem after the addition of pigments. Can you help?

John Woon (Senior Latex Consultant): While some pigments increase the stability of latex, 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. Stir the latex compound gently for about two hours before taking samples for the tests.

If your tests confirm that the pigments are at fault, you should start evaluating pigments from other sources. But more importantly, you should ask your vendors whether their pigments contain “glycol compounds” that could destabilize the latex. Glycol and its derivatives are commonly used in making some pigment dispersions or pastes.

Needless to say, the pH of the pigment pastes must be adjusted before use.

pH Adjustment for Latex Compounds

Manufacturer: Whenever we adjust the latex TSC (Total Slid Content), we encounter an increased formation of coagulum. Why is this so?

John Woon (Senior Latex Consultant): Sometimes, in diluting the latex with water, the latex would get a “pH shock” due to the comparatively low pH (7) of water. Such a pH shock could lead to the formation of micro-coagula. Therefore, it is always advisable to adjust the pH of water with ammonia or KOH to match the pH of the latex before use. There are also other causative factors, such as the laxity in latex specification.

Likewise, the pH of other compounding ingredients must be adjusted before use.

If you want a more comprehensive report on quality improvement in your process, please let me know.

Addition of Diammonium Phosphate (DAP) into Natural Rubber Latex

Manufacturer: We are a producer of latex concentrate.  Our technical team members are arguing about how much diammonium phosphate should be added to the field latex to remove the naturally occurring magnesium. Your input here will be highly appreciated.

John Woon (Senior Latex Consultant): The question of how much DAP is required has been debated over many years with many different schools of thought.  One school of thought expresses concern that, if all the magnesium in the latex has been removed with DAP, the phosphate ions released through hydrolysis of the inherent phospholipids could destabilize the latex, since there is no residual magnesium left to react with.

They therefore suggest that the actual amount of DAP to be used should be only about 80% of the calculated stoichiometric level. On the other hand, other schools of thought actually suggest that the addition of DAP should be about 5% more.

You should conduct experiments to determine what the best solution for your own situation is.

Colloidal Stability of Natural Rubber Latex and the Importance of Latex Specifications

Manufacturer: We are facing a lot of latex stability problems with natural rubber latex concentrate. This occurs intermittently during the year. What is your best advice to overcome this?

John Woon (Senior Latex Consultant): You have to work very closely with your suppliers and the latex producers to decide on the right parameters for the specification of the base latex concentrate.

The producer's specification, though acceptable to others, might not be suitable for your process. This is a big subject, but briefly, here's my advice. You should study the significance of the main parameters, such as MST, VFA Number, KOH Number, and coagulum level, on your processes. You should carry out some of these tests yourself instead of relying totally on the specification from the suppliers/latex producers.

The stability problem is particularly worse during the wintering months of the rubber trees, and extra care and attention should be paid to the parameters of the specification.

Like I said, this is a big subject that is beyond the scope of this blog. If you want, I can write you a comprehensive guidebook on how to monitor and control the colloidal stability of the incoming latex concentrate, the latex during and after compounding, and during processing, such as latex dipping, etc.

Properties of Prevulcanised Natural Rubber Latex (PV) on Storage

Manufacturer: If we make our own prevulcanised latex (PV), how long would it last, and what would happen to the physical properties of the dried film and the latex stability after storing the latex for some time?

John Woon (Senior Latex Consultant): A well-formulated PV could last up to 6 months or more. The physical properties of the dried film remain consistent throughout the shelf life of the PV. However, like the base latex concentrate, the colloidal properties and latex stability will deteriorate upon prolonged storage. But all is not lost. It is recommended to carry out tests on latex stability, such as MST, viscosity, and cagulum content, periodically. Further addition of stabilizers (soaps) and KOH can then be added as and when required, accordingly

Shelf-Life of Natural Rubber Latex Compounds

Manufacturer: We are always plagued with the problems of our latex compounds being over-cured after only a few days of processing. We are now deciding whether we should venture into using prevulcanised latex. But what is the shelf-life of a typical prevulcanised latex?

John Woon (Senior Latex Consultant): The problems you are facing are very common among latex product manufacturers who are using what I call post-vulcanisable latex compounds. Depending on the compound formulations, the latex compounds usually last only for a few days.

It is a good idea to look into switching to the use of prevulcanised latex (PV), which can last up to 6 months. However, the latex film from PVs could be tackier and darker in colour.

If you need full information on how to produce prevulcanised latex, do contact me again at woonsungliang@yahoo.com.sg

Extractable Protein Levels in Latex Products Made from Prevulcanised Latex and Post-Vulcanisable Latex Compounds

Manufacturer: What is the difference in extractable protein levels in the finished latex products made from prevulcanised latex (PV) and post-vulcanisable latex compounds?

John Woon (Senior Latex Consultant): Many manufacturers find that their products made from PV have lower extractable protein levels when a post-oven leaching is carried out, compared with those made from a typical post-vulcanisable latex compound. This is mainly due to the higher degree of controlled porosity in the dried PV film.

Prevulcanised Latex and Its Applications

Manufacturer: Can we use one single grade of prevulcanised latex for more than one applications?

John Woon (Senior Latex Consultant): Yes, one specific grade of prevulcanised natural rubber latex can be used to manufacture more than one latex product. For example, one grade designed for gloves can also be used for making catheters, with minor or no modification of the formulation.

The Role of Free Magnesium in Natural Rubber Latex Stability

Manufacturer: We are both producers of NR latex concentrate and latex-dipped products. Recently, we have had some debates among ourselves regarding the role of magnesium in the latex in terms of its colloidal stability. Will appreciate your simple advice.

John Woon (Senior Latex Consultant): The presence of magnesium in the final latex concentrates can lead to instability of the latex because it can react with the protective stabilizers (soaps) around the rubber particle surface, rendering them insoluble. Soaps must be in a soluble form for them to work.

The residual magnesium in the latex concentrates can also react with the naturally occurring phosphate in the latex. The phosphate is the result of the hydrolysis of phospholipids in the latex.

In some cases, it can form “magnesium crystallites” that could weaken the dried latex film, causing weakness of the rubber film and the formation of pinholes in condoms, toy balloons, and medical gloves.

The problem is that the ratio of magnesium to phosphate in the latex varies from clone to clone and sometimes within the same clone. And the problem of magnesium is particularly bad during the “wintering” periods.  

Manufacturer: We are a small manufacturer of insulation gloves. We are encountering a lot of coagulum problems even after filtering the latex. Our latex specification has been sent to you. Please help us.

John Woon (Senior Latex Consultant): The MST should be a range, not a minimum number, such as 650 seconds. If it is set at a "minimum", the latex concentrate producers are free to deliver to you batches of latex with too high an MST, which is also no good (e.g., 2000 seconds)

Your latex concentrate is almost four months old. The MST today will be very much lower than the 650 seconds tested by the producer, because the stability of latex drops with storage time. 

The lower the MST, the lower the stability of the latex against shearing, such as during stirring and compounding. The end result could be what you are experiencing now, i.e., high coagulum level (evident in the photographs of your gloves)

I suggest you set a range of between 900 and 1500 seconds as your new MST specification.

For the interim, please add 0.05 phr (parts per hundred of dry rubber) of ammonium laurate soap to the latex, followed by stirring for two hours at low rpm.

Then filter the latex through 100 mesh sieves before use. If you still encounter the formation of coagulum, you can add another 0.05 phr of the soap.

                                                                                                                                         

Organic Fillers for Latex Products

Manufacturer: We are familiar with inorganic fillers (i.e., calcium carbonate and clay), but what are organic fillers? What do they do? We are thinking of evaluating organic fillers if we have more knowledge of them.

John Woon (Senior Latex Consultant): Organic fillers normally used in the latex industry are polymeric in nature. Some examples are MMA grafted NR latex, and high-styrene SBR latex.

For some high-styrene SBR latex added in NR latex compounds, the tensile strength initially increases marginally before dropping drastically as the amount of SBR increases. Needless to say, the elongation at break drops while the modulus and hardness increase. The main reason for using high-styrene SBR is to improve the tear strength of the latex products.

MMA grafted NR latex behaves quite similarly as high-styrene SBR latex.

The other advantage of organic fillers is that one can increase the modulus and stiffness of latex products without having to add inorganic fillers such as clay and calcium carbonate, and hence the tensile strength could be easier to maintain.

Fillers for Latex Dipped Products - Tensile Strength

Manufacturer: We are attempting to improve the tensile strength of our latex-dipped products by adding various fillers such as carbon black into our latex compounds but no avail. The products become harder and in fact, the tensile has dropped. Any quick advice?

John Woon (Senior Latex Consultant): I'm not surprised with your findings at all. 

In reinforcing of rubber in the dry rubber technology, there is a process called "mastication" where the raw rubber is first softened using mechanical force (i.e., two-roll mills or internal mixers), by breaking down the rubber molecules into smaller units.

As a result, rubber-bound free radicals are formed that can interact with the reactive sites of the filler particles, hence reinforcing the rubber.

No mastication process is involved in latex technology. Therefore, generally, one cannot increase the tensile strength of latex products. Also, filler will increase the rubber modulus resulting in an increase in hardness.

However, if you want to increase the tear strength, you can try adding a small amount of fine-particle-size silica.

Having said that, some manufacturers do manage to increase the tensile strength marginally with a small amount of bentonite clay (at about 5 to 8phr), but such addition tends to increase the latex compound viscosity. Also, it is very much more expensive than kaolin clay and it is darker in colour.

Temperature of Latex and Its Effects on Latex Stability and Skinning Tendency

Manufacturer: We always encounter some problems with latex stability, especially skinning during summers. Why?

John Woon (Senior Latex Consultant): This question had been raised before (see my earlier post below). When the ambient temperature increases during summers, the temperature of the latex invariably also increases (unless there is a good cooling system).

When this happens, the kinetic energy of the rubber particles is increased and as a result, the rubber particles tend to move at a very much faster rate. This results in the increase in both the frequency and violence of the collision of rubber particles with each other. The end result is coalescing and coagulation. 

Also, at elevated temperatures, the latex surface tends to lose more moisture and ammonia, resulting in skinning. 

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

You can also overcome the skinning problem by adding 0.1 to 0.3 phr of KOH to the latex with the addition of stabilizers such as ammonium laurate to further stabilise the latex.

 

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).