Micro Spheres also known as micro balloons is a filler used in epoxy
Micro balloons or micro spheres are microscopic hollow sphere used as an additive to many materials to modify the characteristics. In boatbuilding it is a thickener in epoxy mixtures particularly useful as a fairing compound.
What are Micro Spheres or Micro Balloons?
Photo credit: Nikgupt from Wikipedia.
As the name indicates they are small hollow spheres. They can vary in size considerably depending on the intended use. Sizes between 1 micrometer and 5 mm have been produced although most microsphere for boatbuilding will be between 20 and 100 micrometer
Microspheres can be made of several materials. Glass and glass variations are common and have a white appearance. Phenolic microspheres are reddish brown. Acrylic microballoons are also produced and are white.
When microspheres are added to a ceramic, metal such as aluminium, or plastic or epoxy substrate the resulting lightweight lightweight material is referred to as a Syntactic Foam (a "made up" foam)
How are microspheres made?
There are several manufacturing processes available to make hollow glass microspheres (HGM). What they all have in common is the use of a substance added to the powdered glass, that creates a gas and this gas is what makes the bubble which becomes the microsphere. The gas producing agent is called a blowing agent. The quick expansion of this blowing agent makes a bubble. One method is to mix a sulfur containing substance often sodium sulphate, with a borosilicate glass containing sodium. This mixture is then heated in a hot flame causing the glass and sodium sulphate to melt. The melting of sodium sulfate and the resultant decomposition of into a sulfur gas, forms a bubble in the melted glass droplets and as they cool, a sphere with a hollow gas filled interior is formed.
Glass microspheres are also produced by acid leaching and further heat treatment of precursor glasses.
Plastic microspheres are made in a similar way as the glass microspheres. The microspheres consist of a polymer shell such as acrylic encapsulating a gas. When heated the internal pressure from the gas increases and the thermoplastic shell softens, resulting in a dramatic increase of the volume of the microspheres and the formation of a gass filled hollow plastic bubble.
Are Micro spheres dangerous to use?
Material data sheet for phenolic microsphere from Polysciences, Inc. Classifies the microspheres as NON-Hazard. They go on to specify the use of gloves and protective eye wear.
As in all light easily disturbed material you should avoid breathing the dust, both while mixing and also when sanding. A good particle dust mask should be used.
Although they are not chemically very active, microspheres can be irritating to the eyes, skin, mucus membranes and should not be breathed in.
The Characteristics of materials filled with microspheres
Micro spheres or micro balloons, are lightweight and when added to a mixture, will reduce the weight. It increases the strength to weight ratio.
Microballoon mixtures also reduce the coefficient of linear expansion making the composite more dimensionally stable.
Radar Transparency can be increased with the use of microspheres. In other words the object is less visible to radar.
Adding microballoons also modifies the electrical conductivity and dielectric characteristics of a material.
The addition of microsphere helps control shrinkage of moulded products. It also adds insulating qualities.
Plastic mirospheres have some give and will bounce back from stress while glass microspheres are more likely to shatter.
Some uses of Micro Spheres
Microspheres are widely used in industry. Anywhere a lightweight non reactive filler is required microspheres are a good choice. They end up in lightweight concrete, and as a stabilizer and strenghtening agent in foams. These foams can be plastics, metals or ceramic based and have found widespread used in underwater floats, between skins of submarines and ROVs. Modifying the density of the microspheres modifies the compressive strength, denser=stronger.
In order to favor various characteristics such as strength, insulation properties, or density, the size of the microballoons and thickness of the wall, can be modified.
Here is an article in Wikipedia on the use of microspheres. It also touches on the different kinds of microsphers.
Plastic Microspheres are getting a bad reputation for getting in the environment as waste from cosmetic and such uses as toothpaste or cleaning products where they are used as mild grit.
Some microspheres can be heated after they have been used, and this causes a significant amount of expansion. This is put to use in such aspplications as puff inks, where the writing is puffed up to give it a textured 3D effect.
Microspheres are also used to provide slow release mechanism of medicines.
Microspheres have been added to paints and have been shown to help keep heat in by reflecting the heat back into a room thus acting as a sort of insulator.
Why is it added to epoxy in boat building?
First and foremost the use of micro balloons helps make a relatively easy to sand mixture and it is widely used as a fairing compound. Photo of 3M glass microspheres. It is not as fly away as silica and it has a slightly gritty feel.
It is also used anywhere a large radius fillet is needed to back up a fiberglass layer to reduce the need of glass to bend sharply. In this case the bulk of the strength will be provided by the fiber glass and the fillet weight and strength can be reduced.
Microballoons are not great thickeners and fumed silica is often added to improve the texture and flow.
All microballoons will make a lightweight easily sanded fairing compound, or non structural fillets. The ability to feather or spread very thinly varies with brands and with the mixture proportions.
Microballoons are useful to speed the filling of fiberglass weave. The filling mixture gets spread with a plastic spreader and saves quite a lot of time filling and sanding un-thickened epoxy.
Acrylic microspheres are more lightweight than glass and cheaper than phenolic microspheres. Some solvents will affect them as will heat but that is not a significant issue in epoxy encapsulation. Douglas and Sturgess supplies it. I think this is also a component of Microlight from West systems.
There are many kinds of microballoons for epoxy work, which should I use.
Photo of brownish phenolic microsphere. It has an oily feel and can be shaped. It tends to stick together until mixed with epoxy.
Phenolic microballoons are a deep red-brown colour. Besides being an excellent sanding filler, it can be used to modify the colour of mixtures.
All microballoons will sag if applied too thickly. It takes a lot more microballoons to thicken an epoxy mixture than if using silica. For this reason a small amount of silica is often added to reduce the sagging of a fairing mixture.
Silica thickens the mixture but is thixotropic which means it is less viscous while being mixed and then thickens after being applied and allowed to sit for a short while.
Adding some wood flour to the mixture used for lightweight fillets will also sometimes improve the texture.
When mixing glass and phenolic microspheres be gentle and mix only till incorporated. It is possible to break the bubbles and make them less effective as a sandable filler.
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