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Home  > Raw materials & technologies  > Applications  > Protective & Marine coatings  > Keeping nuts and bolts without any corrosion

Sunday, 23 February 2020
Raw materials & technologies, Applications, Protective & Marine coatings

Keeping nuts and bolts without any corrosion

Thursday, 25 October 2012

Boats and ships are constructed with nuts and bolts. Corrosion here could weaken the whole stability. Good to know that a new patent shows promising results preventing these small items from corrosion with a special coating.

A new patent shows good corrosion resistant results with a fluoropolymer coating

Source: Liebisch

A new patent shows good corrosion resistant results with a fluoropolymer coating

Source: Liebisch

Xuepu Mao from China and colleagues from  E.I. DuPont de Nemous as well as Jeffrey Hugh Hamilton from Lincoln University invented an aqueous coating compositions for metal substrates based on

  • a functional acrylic polymer,
  • melamine resin,
  • perfluorinated polymers and
  • dispersing polymers.

Their patent utilizes a functionalized acrylic polymer and a melamine resin to provide the main binder component of the coating; a perfluorinated polymer which functions as a dry lubricant in the dry coating, and a dispersing and stabilizing resin to aid in fluoropolymer compatibility in both the wet formulation for formulating and shelf stability, and also in the dry coating in order to maximize corrosion resistance.

This aqueous fluorpolymer composition provides improved corrosion resistance compared to conventional aqueous corrosion resistant coatings.Therfore it is useful for fasteners, such as nuts and bolts, used in marine environments. It also maintains both good coating-substrate adhesion and the ability to release (coating-coating release) so that the nuts and bolts can be unscrewed, even after exposure to salt water environments.

A new kind of perfluoropolymer resins

Fluoropolymer resins, and especially perfluoropolymer resins, are known for their low surface energy and non-stick properties as well as thermal and chemical resistance. However, fluoropolymer coatings often show lower hardness and are not generally suitable as the sole binder in the most demanding seawater applications because they fail to provide sufficient corrosion resistance.

Moreover, perfluorinated polymers are difficult to incorporate homogeneously with co-binders into both the wet aqueous formulation and also into the dried coating. Incompatibilities in the binder composition generally have an adverse effect on the corrosion resistance properties.

It has long been desirable to achieve superior corrosion resistance for metal substrates combined with the release properties associated with fluoropolymers. Ofparticular concern to achieving coated metal substrates with longer service life in seawater applications is the coated substrate's ability to withstand corrosion.

Tests show improvements

The researchers applied the paint by spraying phosphate treated cold rolled steel panels that had been washed to remove grease and then grit blasted.

The coating was permitted to dry by forced air drying at a temperature of 120° C for 15 minutes, followed by cure at 230°C. for 25 minutes.

The dried coating thickness (DFT) of the applied coating was approximately 20 microns (+/-1 micrometer) as measured with a film thickness instrument, e.g., Isoscope, based on the eddy-current principle (ASTM B244).

The test panels are placed in enclosed Salt Fog Spray Test apparatus for a period of 1,000 hours, 1,500 hours, and 2,000 hours, and, after each time period, they were removed, dabbed with a dry cloth to dry, and then visually inspected to evaluate for rusting.

Further tests were done as well:

  • Coefficient of the friction test
  • Pencil hardness test
  • Crosshatch adhesion test (ASTM D3359)
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