We all use latex and nitrile thin gloves. Over 180 billion of them are manufactured each year. The method of manufacture involves taking a ceramic former and dipping it into a solution of electrolyte, typically calcium nitrate. The coated former is dried and then dipped into a bath of dilute, aqueous polymer latex. The calcium nitrate (called coagulant) causes the latex particles to aggregate onto the former forming a weak, water containing film described in the industry as wet gel. Upon removal and subsequent drying the film transforms into the glove, which is then removed from the former and packaged. We sketch the process in the first figure to the right.
We have recently proposed a model where the transport of coagulant into the latex bath is driven by diffusion. The wet gel thickness is experimentally found to be thinner than that predicted and, after considering many mechanisms for the discrepancy, we concluded that a reaction between calcium ions and anionic surfactant ions from the latex reduces the calcium concentration and hence the wet gel thickness.
Despite its industrial importance, many aspects of thin glove manufacture are not yet understood. The application of a better understanding could reduce the environmental impact of the process. For example, to obtain a thin final film, a large amount of water is used in the initial stage to dilute the latex. At the final stage of production, this water is evaporated with a high energy cost.
A) The next step is to extend the modelling beyond the stationary former case examined so far in the laboratory. In industry, the former is moving during deposition at approximately 0.2 ms-1. Consequently, we expect convection to be a relevant mass transfer mechanism. The problem to solve is sketched in the second figure to the right.
There are numerous questions to answer here.
- Using a simple (eg rectangular) former, what is the effect of the movement of former relative to liquid on film build up?
- If there is an effect, can we model it?
- If there is no effect of movement on build-up, can we explain why not?
- Can we extend any movement effect to a real hand shaped former? What is the flow profile around the former? Are there stagnation points?
- Are there any flow instabilities between the fingers?
- When the wet gel starts to form, what is the appropriate boundary condition between the wet gel and latex?
B) As well as the fluid mechanical questions outlined above, we need to examine the diffusion-controlled mechanism further. For example, considering the need for a thin final film, the diffusion-controlled build-up is rapid with the currently used Ca based coagulants. How can we reduce this build-up rate?
- What is the appropriate mass transfer boundary condition for coagulant?
- Is it c = csat at the fluid solid boundary? (presumably for easily dissolved coagulant)
- Is it k dc/dx = const ? (for a slow dissolution?)
- Is it something different related to a dissolution rate?
- Are there alternative coagulating systems with both reasonable cost and low toxicity?
- We also need to examine the reaction between Ca2+ and surfactant anions. Using industrially sourced surfactant, we will experimentally answer
- What is the reaction order?
- What is the rate constant?
- When these data are generated and incorporated into the modelling, what results do we obtain compared to experiment?
C) Once we have a refined model, we will be able to predict the glove thickness and rate of formation as a function of all process parameters. We will then apply our knowledge to a production plant and see how to optimise the process. In particular we seek to minimise the energy input to the plant. This is likely to involve using a latex dispersion with as high a polymer content as possible consistent with producing a thin (
- For a given set of process parameters what is the energy requirement?
- What are the optimum parameters?
- For higher latex volume fractions how does the glove thickness change?
- Are there other processing issues associated with higher volume fraction latex?
Interested students should contact either Alex Routh (firstname.lastname@example.org) or Bob Groves (email@example.com). This project does not yet have funding associated with it. We will seek scholarships if a suitable student is found.