Fiber-reinforced and mineral-filled compounds are not always
compatible with use in dynamic applications, where parts slide, rotate, pivot, or move
across each other. "When a glass-filled part rubs against a metal surface or even a
similar glass-filled surface, rapid abrasion takes place," says George Osterhout,
v.p. of Dimension Bond, a Chicago-based supplier of a specialized coating service that can
address this problem.
"The reason for the abrasion is minute ends of the fiber
reinforcing materials protruding from the surface of the filled part," adds
Osterhout. The sharp ends of the fiber can fracture and cut into the mating surfaces. This
can restrict relative motion between the parts, and the part surfaces may instead
"cling" to each other.
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| Support for actuator rod in
rack-and-pinion steering mechanism was converted from powder metal (right) to reinforced
plastic (left). Bonded coating on the saddle of the support prevents abrasion and reduces
friction between metal rod and glass fibers in the part. |
Several methods have been used to counteract the roughness of
the filled parts to eliminate their abrasive tendency. Inserted bearings or wear pads
between the surfaces have been successfully used, but they add cost. Coatings based on
fluoropolymers such as PTFE, FEP, or PFA have been tried, but there have been problems of
poor adhesion and resulting delamination of the protective film from the part. Other
coatings with sufficient strength to resist abrasion can require a high curing temperature
that can degrade the filled plastic part in the process. Coatings that show better
adhesion or lower cure temperatures have been tried, too, but have been proven to be too
soft to resist abrasion, says Osterhout.
Multi-layered protection
Dimension
Bond developed a special two-layer composite that it applies as a liquid to the
plastic part and cures to a highly adherent, low-friction, abrasion-resistant film.
(Osterhout uses the word "film" instead of "coating" to describe his
products to distinguish them from prior coatings that delaminate.) The new T-series bonded
coating, available for about a year, reduces friction and covers the protruding fibers.
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| Projecting fibers in
reinforced plastics (left) can be very abrasive when the parts slide or rotate against
other plastic or metal components. The T-Series coating from Dimension Bond (right) covers
up the fibers with a low-friction top layer and a bonding underlayer that contains
wear-resistant nano-particles. |
The two-layer coating is composed of a top layer containing
bronze particles to provide a coefficient of friction ranging from 0.05 up to 2, depending
on formulation. The bronze also adds two to 10 times greater wear resistance than an
unprotected part. The under-layer consists of specially formulated nano-particles in a
resin matrix that bonds to the fibers and the plastic matrix and also brings lubricity and
high wear resistance. The nano-particles and resin matrix are proprietary developments
that reportedly can be formulated to work with all known filled resins and engineering
materials, including PBT, PPS, PET, PPO, PEEK, PC/ABS, acetal, nylon, and epoxy.
Unlike other coatings, the bonded film is said to pass ASTM's
3349-4B adhesion test for plastics. In addition, Dimension Bond says the lubricity is
guaranteed for the life of the part.
Proprietary curing process
The film begins as a liquid that is applied and cured on the part. "The film grabs
the glass (or fiber) particles, and also flows into the pores of the polymer. There is a
chemical reaction with the part surface," Osterhout says.
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| Dimension Bond's two-layer
protective coating guards the rubbing surfaces of this valve body and its inner, rotating
component, both of which are a glass-reinforced plastic. |
Dimension Bond applies and cures the liquid coating system in
a proprietary computer-controlled process that takes 1 to 3 hr. The part must withstand a
minimum of 300 F for 30 min to achieve the proper bond. The process creates a film 0.0005
to 0.003 in. thick depending on requirements and on how far the fibers protrude from the
part surface. "The rule of thumb is that films are applied to cover the filaments
plus about 50% extra thickness for insurance," he says . The coating is applied only
to the required area of the part. Its wear resistance can be tailored from that of
bronze/PTFE to four times that level.
Dimension Bond offers the T-Series in a high-temperature
composite line that can be used in applications operating at up to 650 F. A
low-temperature line works in environments from 300 to 400 F. Dimension Bond also produces
a two-layer film coating with a nonmetallic top layer that contains PTFE. It also
formulates a lower cost, single-layer bonded coating that can contain an anti-wear agent
such as PTFE.
Current applications of these protective films include
automotive steering racks, where glass-filled plastic parts have replaced powder-metal
bearing cradles. Other uses are valve actuators, moving parts of electrical switchgear,
and fluid-handling applications where valve bodies, plugs, or sealing surfaces have been
converted from metal to a glass-filled polymer.
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