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ACRYLICS
Acrylic
thermoplastics are products of polymerization of acrylic estermonomers.
The materials are available in cast sheet, continuous cast and extruded
sheet, film and a number of compounds for injection molding and extrusion.
Tubes and rods can also be cast or extruded from the monomer or polymer.
Cell-cast
sheet is produced in a variety of sizes and thicknesses.
The largest sheets available are 120x144 in.
Thicknesses range from 0.030 to 4.25 in. Continuous cast material is supplied as flat sheet and in
coils in lengths to 600 feet and widths to 9 feet.
Maximum thickness is ½ in. Acrylic
sheet cast in the continuous process (between stainless steel belts) is more
uniform in thickness than cell-cast sheet. Cell-cast sheet, on the other hand, which is cast between
glass plates, has superior optical properties and surface qualities.
Also, cell-cast sheet is available in a greater variety of colors and
compositions. Cast acrylic sheet is
supplied in general-purpose grades and in ultraviolet absorbing mirrored and
superthermoformable sheet grades.
All
cast grades are strong, stable, weather resistant and thermoformable.
Sheets are also available in transparent, translucent and opaque colours,
and in a variety of surface textures.
Extruded
sheet is produced by forcing Acrylic pellets through a conventional screw
extruder. The Acrylic is pushed
through a sheet die in a continuous process allowing for a variety of widths and
lengths within practical or economical limits.
The
extrusion process is the most economical approach to the production of Acrylic
sheet. However, these economies are
limited to thickness between .060 and 1.25 thick.
Above this thickness, it is more economical to cell-cast or continuous
cast sheet. Extruded sheet is a lower cost alternative to the other
methods and this is reflected in the quality of the sheet. Die lines and distortion are not uncommon.
It is therefore used when its quality is "good enough" and
economics dictate. Extruded sheet,
however, tends to have a "Grain" in the direction of the extrusion,
affecting its physical properties somewhat.
Sheet
coating is available at a premium price to those who object to the scratching
and marring of Acrylic sheet. A
number of coating processes are available at this time.
Coating imparts a "hard" film to the surface of the sheet,
greatly increasing its marr resistance, cleanability, and chemical resistance,
but sometimes reducing thermoforming capabilities.
When
selling acrylics remember that transparency, gloss, and dimensional shape of
acrylics are virtually unaffected by years of exposure to the elements, salt
spry, or corrosive atmospheres. These materials withstand exposure to light from fluorescent
lamps without darkening or deteriorating. They
ultimately discolour, however when exposed to high-intensity UV light below 265
nm. Special formulations resist UV
emission from light sources such as mercury-vapour and sodium-vapour lamps.
Colorants
can be added to produce to full spectrum of transparent, translucent or opaque
colours. Most colours can be
formulated to give long-term outdoor durability.
Acrylics are normally formulated to filter ultraviolet energy in 400 nm
and lower band.
Mechanical
properties of acrylics are high for relatively short-term loading.
High stresses can be sustained safely for short periods; but for long
service, tensile stresses must be limited to 1,500 psi to avoid crazing or
surface cracking.
Acrylic
plastics are among the most scratch resistant of all thermoplastics.
Toughness of acrylic sheet, as measured by resistance to crack
propagation, can be improved by inducing molecular orientation during forming. Jet aircraft cabin windows, for example, are made from
oriented acrylic sheet.
Applications
for acrylic sheet include:
Aircraft
Boat
Mass Transit
Architectural and Protective Glazing
Internally Illuminated Outdoor Signs
Lighting Diffusers and Skylights
Product Prototypes
Demonstration Models
Special
ultraviolet-absorbing grades are used for document preservation in museums and
for various photographic applications.
Specially
formulated acrylic sheet is available for deeply formed components such as tub
and shower units, which are subsequently backed with glass-fibre reinforced
polyester.
Sheet
extruded from the high-impact moulding grade is used for:
Signs
Thermoformed Products
Toys
Glass-fibre Polyester backed components such as Camper Tops, Furniture
and Recreation Vehicle Bodies
Rod
and tube are available in both extruded and cast grades, the selection of which
should be determined by the quality level desired when compared to cost
considerations.
Cast
rod and tube is manufactured from raw monomer, which is cast or poured into
moulds. Once cured, the material is
ground to size and then highly polished and finished dimensions.
Casting
in the more expensive of the two processes but yields a better quality product.
Cat rod is available up to 8" in diameter; tube may be purchased up
to 24" OD. Slab stock may also
be manufactured in this manner, so that thick "sheet" not available by
conventional means may be purchased from a number of independent custom
manufacturers.
Extruded
rod and tube is manufactured by a number of extruders who push Acrylic pellets
through a highly polished extrusion die to produce a finished product.
Die marks and other imperfections make extruded rod and tube somewhat
less desirable. The process is less
expensive, however, opening up many markets for distributors.
Acrylic
plastics are known for their attractive glossy surfaces, in practically any
colour. Problem is that, in large,
three-dimensional parts, acrylic mouldings aren't rigid enough for some
requirements. One answer to the
problem is provided by a process called "rigidizing".
Rigidizing marries a thin thermoformed acrylic shell to a strong
underlayer of glass-reinforced polyester. Adhesion
is excellent, and the results are being seen in successful applications
applications, such as tops for vans and other RV's.
The
lightweight and toughness of clear plastic sheet qualifies it for many jobs
formerly held by glass. Materials in this category include acrylics, cellulosics,
polyarylates, polysulfones and polycarbonates.
Despite the durability of these materials, however, their surfaces are
softer than those of glass, making them more susceptible to hazing by surface
abrasion and to damage by scratching. But
progress is being made; more abrasion-resistant materials and surface treatments
are being developed.