CASTING : Cast Polymer – Introduction & Materials

An excerpt from the10th Edition of the Composites Application Guide by Cook Composites and Polymers

‘Cast Polymer products or the countertop and bath-ware industry, are used in commercial, residential, industrial, and medical areas. The cast polymer product line has extended beyond the original kitchen and bath markets. Examples of cast polymer products are vanity tops, sinks, bathtubs, shower pans, wall panels, countertops, windowsills, flooring, bar sinks, interior and exterior facades, banisters, and furniture, with the list continuing to grow. There are three distinct product lines within cast polymers:
• Cultured Marble—gel coated surface, filled with calcium carbonate, usually pigmented and/or veined, opaque in appearance.
• Cultured Onyx—gel coated surface, filled with aluminum trihydrate, usually veined with a non- pigmented background, and translucent in appearance showing depth like natural onyx.
• Cultured Granite—gel coated surface, filled with specially designed granite filler to produce a multi-colored speckled appearance.
An unlimited variety of colors, cosmetic designs, and shapes that can be manufactured or fabricated provide cast polymer products with distinct advantages over their natural counter- parts. In addition, natural products are porous (which can become a source of bacterial growth) and can easily be stained, while cast polymer products are not porous and are stain resistant.

A. Gel Coat
The use of gel coat is required for cast polymer products of cultured marble, onyx, and granite, The function of gel coat is to protect the part from its environment, providing chemical resistance, water resistance, and weathering resistance (UV stability). The gel coat is also accountable for the part’s cosmetic surface and durability. It is the gel coated surface that is visible and therefore a critical aspect of the part.
Both pigmented and clear gel coats can be used to produce cast polymer products although clear gel coats are generally more popular.
Pigmented gel coats are mainly used to produce cultured marble. Clear gel coats add depth and dimension to the part. Artistic colors and designs, such as veining and granite effect of the matrix are viewed through the clear gel coat. Clear gel coat is also used to produce cultured marble where veining patterns are applied. Clear gel coat is required with onyx to accentuate the translucency and depth of the veining to more closely resemble natural onyx. Clear gel coat is also required to show off the multi-colored effect of granite filler.

Cultured granite can be manufactured using one of two methods:
1) Granite effect filler is mixed into the resin and poured behind a clear gel coat.
2) Specially designed spray granite chips are mixed with a specially designed clear gel coat, and then sprayed onto the mold. Standard marble matrix is poured behind it.
Clear gel coats used in the cast polymer industry are specifically formulated for cast polymer applications. Key differences from clear gel coats formulated for other industries are lower color and slower cure rates. Clear gel coats to be used with spray granite chips also have different viscosity and spray characteristics.
To meet performance requirements of bathware products both pigmented and clear gel coats must be based on ISO/NPG polyester resins. Please refer to Part Four Open Molding for additional information on gel coats.

B. Resin
Casting resin is mixed with fillers to make the matrix. The matrix gives the cast part its structural integrity. Resin suppliers formulate casting resins from several components including the polymer, reactive monomer, promoters, inhibitors and specialty additives. The specific components and amounts used are dictated by the end-use application, manufacturing process, required cure behavior, end-use physical properties, and requirements and manufacturing plant conditions. Plant conditions can dictate that resin gel time and/or viscosity be varied to account for seasonal temperature changes.
Unsaturated polyester polymers are the basis of casting resins. Cultured marble, onyx and granite resins are based on orthophthalic polyesters.
The monomer fulfills two roles in the polyester resin. First, it reacts and crosslinks with the unsaturation sites in the polymer to form the crosslinked thermoset material. Second, it reduces the viscosity of the polymer to workable levels. The most common monomer used in casting resins is styrene.

Promoters, also called accelerators, split the peroxide catalysts used to cure casting resins into free radicals. These free radicals attack the unsaturation sites in the polymer preparing them for reaction with the monomer. Promoters used in casting resins determine the cure behavior and also have a significant impact on finished part color. In general, the higher the promotion level the darker the cured resin color. As a result the types and amounts of promoters used in casting resins vary depending on the production speed and color require -ments for each application.
Inhibitors provide shelf life stability to casting resins as well as help control the working time or gel time. Free radicals generated in the polyester resin during storage or after addition of peroxide catalyst react preferentially with the inhibitors. Only after all the inhibitors are consumed does the crosslinking or curing process begin.
In addition to the above materials, a number of other additives can be used in casting resin formulations to affect properties. These include processing aids such as air release agents and wetting agents. Additives can also be used to affect the product’s performance such as UV absorbers and light stabilizers for weathering performance.

C. Fillers
The filler is the largest part of the cast polymer composition. The type of filler to be used depends on the cast polymer product.
1) Cultured Marble—
a) Calcium Carbonate (CaCO3)—Typical marble filler is calcium carbonate (ground limestone). CaCO3 is mined and ground into small particles; size is measured in units called mesh. Filler particles are sorted through screens with different size openings. Mesh size is designated by the number of holes per linear inch, with lower numbers indicating a coarse or large particle size, and higher numbers indicating a small or fine particle size. CaCO3 fillers are supplied as ‘all coarse’ or ‘all fine’ particles or as pre-blended bags of coarse and fine particles.
b) Dolomite—Just like CaCO3, dolomite is a mined mineral, a mixture of calcium carbonate and magnesium carbonate. It is supplied just like CaCO3. Dolomite is more abrasive than CaCO3 and therefore may require more equipment maintenance.
c) Lightweight Fillers—The use of lightweight fillers in cast polymer products has been steadily increasing. Lightweight fillers are hollow spheres made of glass (silica) or plastic. They occupy space or volume but do not add weight, which effectively reduces the weight of a given part without changing its dimensions. Lightweight fillers are used with CaCO3. They can be bought separately or pre-blended with CaCO3 to a known weight displacement. Typically, these fillers demand a higher resin percentage for wet out and to maintain a flowable viscosity. Also, because of its insulating effect, lightweight fillers cause the exotherm of the curing part to increase.
2) Cultured Onyx
a) Aluminum trihydrate (ATH)—The filler of choice for cultured onyx, ATH is a by-product resulting from the processing of bauxite minerals in the manufacturing of aluminum. Onyx grade ATH is much brighter white than CaCO3, thus eliminating the necessity of using background pigment. It is a semi-translucent granular filler which provides a visual effect like natural onyx, and has the added feature of acting as a flame retardant. At temperatures of 410°F, ATH releases its water particles, slowing combustion and reducing smoke generation.
b) Lightweight Fillers—are not recommended since this would reduce translucency.
3) Cultured Granite Granite effect fillers are also gaining in popularity. Filler suppliers have specially formulated colors and particle size distributions to achieve a multi-colored speckled granite appearance, and to give the product a cosmetic textured look. The colored granules may be coarse ground minerals or synthetically made from pigmented resins. The resin demand will vary greatly depending on the granule size(s) and distribution. There is a difference in granite effect filler mixed into the matrix versus spray granite filler mixed into the gel coat and sprayed; therefore, method of application needs to be noted when purchasing these fillers.

While inexpensive initially, if not chosen and checked properly, fillers can become extremely costly. For example, if too much coarse filler is used and subsequently settles, a resin rich area will result on the back side which could cause warpage. If too much fine filler is used, the viscosity will be very high resulting in air entrapment. If the fillers contain too much moisture or become damp, they will affect the gel and cure.

D. Catalyst/ Initiator
Catalyst is the component needed to ‘harden’ the polyester resin mix into a solid mass. Technically, catalyst causes the reaction but does not participate in the reaction. In the composites industry, the correct term is initiator, which starts the reaction and is consumed by the reaction. There are three common types of room temperature initiators used in cast polymers:
1) Methyl Ethyl Ketone Peroxide (MEKP)—The most widely used initiator, MEKP is a clear liquid that easily mixes into the resin. It is the most cost effective choice, and is available in different ‘strengths’ to give a variety of curing characteristics. Recommended range is 0.5 percent to three percent catalyst level based on resin amount.

2) 2,4-Pentadione Peroxide (2,4-PDO)—Also known as acetylacetone peroxide (AAP), this initiator offers fast cure time and high peak exotherm. Although it does lengthen gel time, Barcol hardness builds quickly. Typically, 2,4-PDO is used during the colder temperatures It is avail- able separately or pre-blended with MEKP; however, pre-blends are the most popular. In pre-blends, the MEKP controls the rate of the gel time and the 2,4-PDO provides the faster cure rate and higher peak exotherm. Recommended range is one percent to two percent catalyst level based on resin amount. Above two percent, 2,4-PDO peroxide may inhibit cure. The only disadvantage with this initiator is that with some resins, cured casting color may have a yellowish tint.
(3) Cumene Hydroperoxide (CHP)—CHP lowers peak exotherm and lengthens gel and cure times. Lower peak exotherm reduces cracking, crazing, and shrinkage but also slows down Barcol development. CHP is most popularly used during hotter temperatures and/or on thick parts like shower pans and tubs. It is available separately or pre-blended with MEKP; however, pre-blends are the most popular. Some control over gel and cure rates can be achieved by changing initiator levels and blending the above mentioned initiators.

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