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Latex dipping

Latex dipping is a technique used in industries based on the manufacturing of thin walls or thin-film latex products. Mainly gloves, balloons, condoms, masks are manufactured by the latex dipping process. Simply molds or formers are dipped in the compounded latex and form a thin layer of the latex. The thickness of the deposit produced can be altered by changing various parameters such as viscosity of the compound, dwell time, number of dipping, and use of a coacervate, etc. 

The compounded latex is still in wet form, and it is stabilized in its dispersion medium (water). When making products, the dispersion medium should be removed. In other words, the electrostatic stability of the latex is collapsed, and coagulation occurs. (Latex destabilization). The latex destabilization can be carried out by either physical or chemical methods.

Latex destabilization - Physical methods

In physical destabilization methods, the dispersion medium is removed by heating, freezing, evaporation, or mechanical agitation.

The dispersion medium is removed by evaporation
Figure 01: Dispersion medium is removed by evaporation

Latex destabilization - Chemical methods

In the chemical method, specific coacervates are used. this can be described under three groups.

01. Direct coacervates

Materials that would bring about immediate destabilization as soon as they are added to NR latex. Therefore, the direct coacervates do not add to the latex compound. The mold or former is first dipped in direct coacervates and then dipped in compounded latex.

As the instant destabilization of latex, a thin layer of latex is obtained. Acidic substances, metallic ions, cationic surfactants, water-soluble organic matrix, and rubber soluble organic matrix are used as direct coacervates.

02. Heat sensitizing coacervates

Heat sensitizing coacervates destabilize the latex when they are applied heat. These coacervates are added to the cold latex compound. The mold or former is first heated and then dipped in the latex compound. Therefore, on the mold, a thin layer of latex film is formed. Three principal systems can be used for heat sensitization of NRL.

  • Zinc ammine system
Zinc ammine system in heat sensitization of NRL in heat sensitization of NRL
Figure 02: Zinc ammine system in heat sensitization of NRL

  • Polyvinyl methyl ether system (PVME)

PVME is soluble in cold water but insoluble in hot water. The ability of PVME to heat sensitize the NR latex is a result of this behavior.

Polyvinyl methyl ether system (PVME) in heat sensitization of NRL
Figure 03: Polyvinyl methyl ether system (PVME) in heat sensitization of NRL

  • Polypropylene glycol system (PPG)

PPG is also a non-ionic hydrophilic polymer soluble in cold water, but insoluble in hot water as PVME. The mechanism of heat sensitization given by PPG is related to this solubility behavior.

Polypropylene glycol system (PPG) in heat sensitization of NRL
Figure 04: Polypropylene glycol system (PPG) in heat sensitization of NRL

03. Delayed action coacervates

Delayed action coacervates take some time to take place in the destabilization process. In the foam manufacturing industry, delayed action coacervates are widely used. 

Latex viscosity vs time
Figure 05: Latex viscosity vs time
Delayed action coacervation mechanism
Figure 06: Delayed action coacervation mechanism

Principal Dipping Processes

01. Straight dipping

Straight dipping also known as simple dipping is carried out without the assistance of a coagulant (coacervates). The formation of the deposit depends on two factors which are uniform wetting of the former by the latex and the viscosity of the latex. The thickness of the rubber deposit obtained in one straight dip is usually around 0.05 mm.

02. Coagulant dipping

Widely used technique in the dipping industry. The principal advantage of this process is, it produces considerably thicker deposits than the simple dipping process. The thickness of the deposit obtained in one coagulant dipping is around 0.2 – 0.8mm. The process can be divided into two methods.

Wet coagulant dipping

Wet coacervates such as acetic acid are used. This can be done in two ways.

The former is first dipped in the coacervate

This type of wet coagulant dipping is not widely used. The disadvantages of this process are the non-uniformity of the deposit and the tendency of the deposit to slip off the former during the withdrawn. 

This slippage is due to the liquid layer in between the deposit and the former. The liquid layer formed partly from the wet coagulant and partly from serum.

The former is first dipped in the latex compound

The mold is first dipped in the latex compound and then withdrawn. Next, it is dipped in the coagulant and withdrawn. Again, the mold is dipped in the latex. The former is allowed to dwell in the latex for a predetermined time to get the required thickness of the deposit. One disadvantage of this second method is that it involves two latex dipping.

Thickness variation with dwell time
Figure 07: Thickness variation with dwell time

Dry coagulant dipping

The most widely used method in coagulant dipping is the dry coagulant dipping method. In the latex industry, CaCl2, Ca(NO3)2 are used as dry coagulants.

Multiple coagulant dipping

It is possible to further layers of rubber formed by repeated coagulation and latex dipping. This multiple dipping method produces thicker films. Also, it is possible to make films containing more than one type of rubber. Sometimes, the inner layer may be produced from NR to take advantage of elasticity and strength whereas the outer layer may be produced from a synthetic rubber that has higher oil and chemical resistance.

Preparation of coagulant solution

 IngredientsParts by weight
CaCl215
Ca(NO3)215
(IMS) industrial methylated spirit50
Water20
Wetting agent (nonionic surfactants)0.001 (additional)
Table 01: Preparation of coagulant solution

In addition to the above salts parting agents such as CaCO3, talc at the level of 1.5% by weight of the level of coacervates can be used to facilitate subsequent dry deposit on the former.

03. Heat sensitizes dipping

A heated former and a heat-sensitive compound is used. The thickness of the deposit depends on the degree of heat sensitivity of the latex compound, the temperature of the former, and the heat capacity of the former. The most widely used heat sensitivity agent is PVME. A single heat-sensitive dip can produce a latex thickness of up to 4mm. This process is used in the production of thick-walled articles such as baby teats and electrician gloves.

Thickness variation with dwell time in dipping processes
Figure 08: Thickness variation with dwell time in dipping processes

Compounding for latex dipping 

Compounding ingredientParts by weight
Latex 60%100
KOH 2050.2
Potassium laurate 20%0.4
Sulfur 35%1.2
ZnO 50%1
Accelerator 50%1
ZMBT 50%0.2
Antioxidant 50%1
Table 02: Compounding for latex dipping

Defects in dipped articles

  • Pinholes – pinholes in the dipped products are formed due to the air bubbles attached to the former. These air bubbles make a thin spot of the latex and eventually it will become a pinhole. This defect cannot be completely avoided.
  • Webbing – sometimes the uneven latex films are obtained. (Thin films)
Defects in dipped articles
Figure 09: Defects in dipped articles

  • Mud cracking – in some situations the surface of the latex products looks like deep and irregular mud cracks.
  • Black spot or stain – generally, natural rubber latex is white. But sometimes black spots or staining has occurred in some products.

Products manufactured by the latex dipping process
Figure 10: Products manufactured by the latex dipping process

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References and Attributes

Figures:

The cover image was designed by using an image by Marketingdepascale, licensed under CC BY-SA 4.0, via Wikimedia Commons

Figure 10: Image from balloonhq.com - How Balloons are Made


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