When it comes to dip molding products with emulsions of liquid rubber, it is necessary to complete a series of process steps to assure proper formation, vulcanization and finish treatment to meet the customer’s needs in the final application.
Mark Agee, Kent Elastomer Products
Dip molding can enable the creation of durable medical device parts in a variety of shapes, sizes and wall thicknesses, including probe covers, bellows, neck seals, surgeon gloves, heart balloons and other unique parts.
Natural rubber has outstanding resilience and high tensile strength but also carries a protein that can cause an allergic reaction in humans. Synthetic neoprene and synthetic polyisoprene, in contrast, are non-allergenic. Neoprene stands up against a multitude of factors; it’s resistant to flame, oil (moderate), weather, ozone cracking, abrasion and flex cracking, alkalis and acids. Polyisoprene is a close replacement to natural rubber when it comes to feel and flexibility, with better resistance to weather than natural rubber latex. Polyisoprene, though, does sacrifice some tensile strength, tear resistance and compression set.
The term “dipping” is associated with the manipulation of the dip form. In fact, the forms are dipped into the materials as the sequence is performed. It is important to ensure rubber recipes meet FDA medical device guidelines and requirements.
Here’s what the process looks like:
The dipping process can be characterized as a conversion sequence: The rubber is converted from a liquid to a solid and then chemically converted into a vulcanized network of molecules. More importantly, the chemical process converts the rubber from a very fragile film into a networked group of molecules that can stretch and deform – and still return to their original shape.
Coagulation: Changing a liquid to a solid
The coagulation process is not always necessary for all “dip” processing but is critical to our processing sequence. The rubber can be allowed to change from a liquid to a solid through air drying, but that will take much time. Some-thin walled parts are produced in this manner. The coagulation process uses chemicals to force this physical state change.
The coagulant is a mixture or solution of salts, surfactants, thickeners and release agents in a solvent, typically water. Alcohol can also be used as the solvent in some processes. Alcohol evaporates quickly and leaves very little residue. Some water-based coagulants will require help from an oven or other means to dry the coagulant.
The main component of the coagulant is the salt (calcium nitrate), an inexpensive material that provides the best uniformity of coagulation over the dip form.
Surfactants are used to wet out the dip form and assure a smooth, uniform coating of coagulant onto the form.
Release agents such as calcium carbonate are used in the coagulant formula to aid in the removal of the cured rubber part from the dip form.
Keys to coagulant performance include uniform coating, fast evaporation, material temperatures, entrance and retrieval speeds, and easy change or maintenance of the calcium concentration.
The rubber dipping step
This is the stage in which the rubber is converted from a liquid to a solid. The chemical agent which facilitates the solidification, the coagulant, is now applied to the dip form and is dry.
The form is “dwelled,” or held immersed in the tank of liquid rubber. As the rubber makes physical contact with the coagulant, the calcium from the coagulant causes the rubber to destabilize and turn from a liquid state to a solid state. The longer the form is immersed, the thicker the wall will develop. This chemical reaction will continue until all the calcium is consumed from the coagulant.
Keys to latex dipping include entrance and exit speeds, temperature of the latex, uniformity of coagulant coating, and controlling pH, viscosity and total solids of the rubber.
The leach dip
The leach process is the most effective stage to remove unwanted, water-based chemicals which are not wanted in the final product. The most opportune time to remove the unwanted materials from the dipped film is the leach before cure.
What is removed? The leach process removes residual salts, surfactants and water based proteins.
Main material components include the coagulant (calcium nitrate) and rubbers (natural (NR); neoprene (CR); polyisoporene (IR); nitrile (NBR)). Inadequate leaching can result in “sweating,” a sticky film on the finished product, as well as adhesion failure and increased risk of allergic reactions.
The keys to leaching performance include water quality, water temperature, dwell time ad water flow rate.
This step is a two-step activity. The water in the rubber film is being removed and the temperature of the oven along with time is activating the accelerators starting the cure or vulcanization process. Cure time and cure temperature are key when it comes to optimizing the best physical properties of the different types of rubbers.
Several options are available to treat the surface of a dipped part so that the part does not stick to itself. Options include a powder part, urethane coating, silicone rinse, chlorination and soap wash. This is about what the customer wants or needs for their product to be successful.
Mark Agee, is manager of customer dip operations at Kent Elastomer Products (Kent, Ohio).