Effect of Surface Tension on Wetting and Foaming of Latex

Manufacturer:  We are using synthetic latex for our products. Can you please explain the relationship of surface tension on wetting and foaming in a latex system and a substrate.

John Woon (Senior Latex Consultant): A latex system is said to have high surface free energy when it does not wet on certain surfaces such as waxes and oily coatings. As a result,  the surface free energy tends to force the latex into spherical shape. The end results is that the initial thin film of latex breaks up to form smaller semi-spherical pools of liquid separated by dry areas of the substrate.

The term "wetting" is often encountered in latex technology. Some surfaces of substrates have affinity for water which creates a force (surface attractive force) greater than the surface tension of the water resulting in good "wetting" and the surface is readily coated by the water.

On the other hand, other surfaces might have a lower affinity for water but greater affinity for air. In this case, air has to be removed to overcome the initial resistance before the surface could be "wetted" by the water.

Emulsion polymerized synthetic latex depends on a soap or stabilizer system for polymerization and stability. The soap is partly absorbed onto the surface of the latex particles and partly dissolved in the water (or serum). These soaps reduce the surface tension by moving to the surface layers and hence lowering the surface free energy. Therefore latex surface tension is always lower than that of pure water.

The extent of the reduction of the surface tension depends on the amount and type of soaps in the serum.
Generally, as the surface tension reduces, wetting (i.e. wettability) improves while foaming tendency increases.

Invariably, all efficient soap systems used in polymerization would foam to a certain extent. However, by judicious choice of the soap type and level, the latex manufacturers should be able to prevent the undesirable excessive foaming.  

Lanxess to Produce First Bio-Based EPDM Rubber in The World

Lanxess aims to commercially produce ethylene-propylene-diene monomer (EPDM) from bio-based ethylene by the end of the year. It will be the first form of bio-based EPDM rubber in the world. EPDM is conventionally produced using the petroleum-based raw materials ethylene and propylene. Alternatively, Lanxess plans to use ethylene derived purely from the renewable resource sugar cane.

This bio-based form of ethylene is produced by dehydrating ethanol from Brazilian sugar cane. The company Braskem S.A. will supply the bio-based ethylene via pipeline to Lanxess’ existing EPDM plant in Triunfo, Brazil. Triunfo currently produces 40,000 metric tons per year of regular EPDM rubber and it is expected that the first batches of the product Keltan Eco will amount to several hundred metric tons. In addition,

Lanxess is already seeking alternative sources to produce the premium synthetic rubber product butyl rubber, which is used predominantly in the tire industry. Together with Colorado-based Gevo, Inc, Lanxess is developing isobutene from renewable resources starting with corn. Isobutene is a key raw material needed in the manufacture of butyl rubber.

Latex Cement Composition

Manufacturer:  We use SBR or XSBR latex as bonding agent and gas block additives in the cementing section. The problem is the XSBR we recently purchased is difficult to mix into a fresh water cement slurry, despite large additions of naphthalene sulfonate dispersant.


John Woon (Senior Latex Consultant): You should try pre-stabilizing the latex with a non-ionic surfactant and Potassium hydroxide first before adding the cement slurry. If you are using Portland cement, switching to albuminous cement would help. Also, instead of using anionic latex, you could use cationic latex.

If you continued to have problems, please contact me at woonsungliang@yahoo.com.sg

Chinese, Malaysian Companies in Joint Venture

The Straits Times/Asia News Network reports that two Chinese companies, Mazhongdu International and Hainan Baisha Industrial, have teamed up with a Malaysian firm, Hateg, for a rubber production joint venture in Indonesia, in what is said to be a 3.5-billion-ringgit ($1.14 billion) investment over the next five years.

The first phase of the project in Kalimantan will cover 1,000 hectares over a year at a cost of 40 million ringgits, while another 40,000 hectares will be planted over five years after that. The joint venture partners are also in discussion to expand the plantation to 200,000 hectares, which will bring the total investment to about 8 billion ringgits.

Probelms with Latex Dipping of Vascular Catheters

Manufacturer:   I am an Engineer from a vascular catheters manufacturer. I found your blog (http://latexconsultants.blogspot.com) during my search for answers to the problems I am facing with my rubber manufacturing process and have found the information on the blog site very useful. However, I still have questions and I hope you will be able to give me some advice based on your technical expertise on rubber manufacturing and many years of experience. This is my first time working on rubber manufacturing and I have little knowledge in this aspect.

I am recently working on my own coagulant mixture of Calcium Nitrate plus Methanol and have used it to dip with natural dipping latex. However, I have been finding extremely bulky tip immediately after the formers have been removed from the latex tank. I have tried adjusting the dipping speed, only to find the same results. I have also adjusted the dipping duration for the coagulant, with similar end results. Is the Calcium Nitrate that I have added too little?

I am also facing balloon recovery issues on my balloon after inflation. What are the possible reasons for recovery problems?

John Woon (Senior Latex Consultant): 

Here's my comment and recommendation:
1) At first I thought what you meant by "bulky tips" was the effect of excessive pick up of either the coagulant or the latex at the tip on withdrawal for the formers from the two respective tanks. However, your photos show that it was the narrowing of the areas just below the tips making the tips appearing bulky and thicker.

2) Anyway, let's assume that it was in fact due to excessive pick up of either coagulant or the latex. To overcome this problem, you have to withdraw the former at a lowest possible speed.

3) Formers should be heated to higher temperature to ensure faster drying of the coagulant. You are now using 37C. Please try 45C. Faster drying of the coagulant would help to "set" it before it has a chance to flow back when the formers are inverted after withdrawal. Excessive flow back might result in thinning of the coagulant and hence latex pick-up as seen in the photos.

4) Ensure the formers are well cleaned (with detergent periodically) after each cycle of dipping. This is to ensure even coating of the coagulant.

5) Add about 0.1% of a non-ionic surfactant as a wetting agent in the coagulant. Triton X100 and Teric 320 are possible examples.

6) For health reason, do not use methanol which is highly toxic. It has been reported that workers who are repeatedly exposed to methanol could suffer from eventual blindness. Replace it  with ethanol or IPA (Isopropyl alcohol)

7) Poor recovery of the balloons after inflation and deflation could be due to insufficient drying of the balloons and also low modulus of the film. Prevulcanized latex is available commercially in three modulus ranges, namely low, medium and high modulus. I suggest you use either the medium modulus grade or a 50/50 blend of medium modulus and high modulus.

Malaysian NR Production Up 17%

Malaysia’s Department of Statistics reported that production of natural rubber for July was 100,281 metric tons, an increase of 14,895 mt (+17.4%) compared to the previous month. For year-on-year, production of NR rose by 23.7% compared to July 2010.

The highest consumption of NR was from the rubber glove industry (65.3%), followed by rubber thread (13.5%) and tires and tubes (7.8%). These three industries consume more than three quarters of the overall domestic consumption in the country with a total of 30,341 mt (86.7%)

Biodegradability of Rubber

Manufacturer: We manufacture a range of moulded rubber products using both synthetic and natural rubbers. How biodegradable are these products? Would appreciate some brief notes from you.
 
John Woon (Senior Latex Consultant): It has been documented that degradation and assimilation of organic polymers by living organisms (i.e. bacteria and fungi) is possible and that natural rubber is more biodegradable than synthetic rubbers. Also, such degradation is facilitated by hydrolytic and oxidation breakdown of the polymers.

Microbiological degradation is favoured by high humidity, adequate minerals, sources of available carbon and absence of light. Past researches had indicated that Actinomyces such as Nocardia and Steptomyces are microorganisms responsible for the degradation of Natural Rubber products.

Thin products such as medical gloves made from natural rubber have been shown to degrade at quite a rapid rate after about 2 weeks of exposure to Nocardia, some experiencing a weight loss of more than 70%.

You could conduct your own test using ASTM D5988 - 03

Alkalinity and pH of Natural Rubber Latex

Manufacturer:  We would like to know about difference between HA & LA type NR latex for its properties like pH , NH3 content etc . If their pH difference is less how NH3 content would be less? Also let us know if ammonia is removed from latex to 0.2% what will be pH.  Please include any further details.


John Woon (Senior Latex Consultant):  Generally, as the ammonia level is reduced either chemically (using formaldehyde) or by blowing hot air over it while stirring, the pH would also drop.

However, the non-rubber constituents of natural rubber latex could sometimes act as a buffer. Hence the pH might change only slightly despite a greater change in ammonia level. Different batches of latex might behave differently for obvious reason.

The best way to confirm this is to conduct lab tests by determining the different levels of ammonia content i.e. the alkalinity using ASTM method and measuring the pH of the corresponding latex samples.

New Rubber Thread Factory in Vietnam

Dak Lak, Viet Nam - The Viet Nam News reports the Dak Lak Rubber Thread Company (Dakruthread) has opened its new rubber thread factory in Hoa Phu Industrial Park, in the central highlands province of Dak Lak. Total investment in the factory has reached VND170 billion ($8.6 million) with a capacity to produce 4,000 metric tons per year. The factory will provide products for the international and domestic garment industry and will minimise imported items from Thailand and Malaysia. 

Latex Particle Size and Molecular Weight of Rubber

Manufacturer: We are a group of university students studying the molecular weight of Natural Rubber.
 Is there a correlationship between rubber molecular weight and latex particle size?


John Woon (Senior Latex Consultant):  While some studies by researchers indicated that the molecular weight of natural rubber is not directly related to the particle size of the rubber particles in latex which normally varies from about 0.02 μm to 3 μm, other studies showed that smaller rubber particles might contain rubber with higher average molecular weight.

This had been confirmed that by measuring the molecular weights of skim rubber and rubber from latex concentrate both obtained from the same batch of latex. Skim rubber which is made from smaller rubber particles showed lower molecular weight.

Also, there is no correlationship between different clones of the rubber trees and the average molecular weight.

Distorted Latex Dipped Products

Manufacturer:We observed our latex dipped products distorted on the formers and also on stripping. We have sent our latex compound for analysis and confirmed that the curatives are working OK. Where could we have gone wrong?

John Woon (Senior Latex Consultant): Assuming your curatives are adequate for the required degree of cross-linking and cure state, I suspect your "maturation" period might not be sufficiently long enough. "Maturation" is to allow for some degree of "pre-vulcanisation" besides allowing the surfactants and soaps to reach an equilibrium state.

If you start dipping without "maturation" or with insufficient maturation, you are bound to have problems of "bagging" or "blowing" or distortion due to weak latex gel strength. Also, the finished products would be under cured despite good cure temperature and time which would also result in distorted and over-sized products during stripping.