Resin

Resin

Resin is the basic building = block of=20 solid surface. In the proper form it endows the product with = enhanced stain resistance, heat resistance and repairability = characteristics, yet its overall effectiveness depends = entirely on=20 the type and quantity of ATH, catalyst and pigment that is = added.=20 Generally it comes in some variation of two basic flavors = =EF=BF=BD=20 polyester and acrylic, a difference which has sparked a = seemingly=20 unending debate over which is better.

For all their many = differences, acrylic=20 and polyester solid surface resins actually have many=20 characteristics in common, especially when compared to the = vast=20 range of consumer and industrial plastics that are created = from=20 their closely related cousins. Unsaturated polyester, for = example,=20 is much more closely aligned to cross-linked acrylic than it = is to=20 saturated polyester, which can be made into film, shirts or = bowling=20 balls. In fact, both resins can be engineered to produce = solid=20 surface sheets that cover the entire gamut of physical = properties =EF=BF=BD=20 from thermoformable to non-thermoformable, or from heat and = stain=20 resistant to brittle and relatively porous.

Acrylic

A petrochemical,=20 methyl-methacrylate, or MMA, is traditionally created = from a=20 chemical reaction between acetone and hydrogen cyanide, = which is=20 then combined with methanol. The mixture undergoes = hydrolysis, and=20 then is distilled to a clear, water-like liquid that is = 99.9% pure.=20 Left alone at room temperature, the MMA will increase in = viscosity=20 until it becomes a hardened lump. This is the natural = process known=20 as polymerization.

"In the old days the MMA had = a tendency=20 to polymerize on its own if not inhibited," explains Ettore=20 Minghetti, Manager of Acrylic Technology for Aristech = Acrylics.=20 "Let=EF=BF=BDs say you take a bottle of non-inhibited MMA. = Within a week it=20 would increase in viscosity because either the light or some = impurities would initiate a polymerization. In the chemical=20 laboratories they keep it cold in the refrigerator and in = the=20 dark."

Today, chemical companies add = inhibitors,=20 which prevent premature polymerization of the MMA monomer, = and they=20 keep it cool =EF=BF=BD about 60 degrees Fahrenheit. At the = solid surface=20 manufacturing plant other chemicals are added to prepare the = resin=20 for casting into sheets. "We take the monomer and do what is = called=20 a pre-polymerization," Minghetti says. "We put in some = catalyst,=20 raise the temperature and start the polymerization. We let = it go a=20 little while, and then stop it."

Perhaps we should pause for a = moment to=20 define some terms. A monomer is a molecule that is = able to=20 link together to form a polymer. The reason it is = able to do=20 this is that it contains double bonds, also known as=20 unsaturation. Monomer (one unit) changes to polymer = (many=20 units). During polymerization, the mixture transforms from a = watery=20 liquid to a rubbery-like material, and finally into a = hardened mass.=20 In at least one variation of the process, the mixture = undergoes=20 pre-polymerization, which means that some polymer molecules = are=20 introduced into the monomer resin, which remain dissolved = until the=20 solid surface manufacturing process begins.

"What you get via the = polymerization=20 reaction is a three dimensional network linking together the = polymer=20 and monomer constituents," says Borys Schafran, Product = Manager,=20 Specialty Methacrylates, at Silikal Resin Systems (SRS), = part of the=20 new Degussa. "You dial in what you want according to the = polymer and=20 the monomer. You can make anything from a very stiff, rigid = acrylic=20 material, like plexiglass, to an extremely soft, flexible = material,=20 such a concrete joint filler."

In the Aristech Acrylics = process,=20 pre-polymerized MMA resin is mixed with ATH, crunchies (if=20 applicable), pigments and peroxide catalysts in the casting = machine.=20 The temperature is raised to between 170 and 190 degrees = Fahrenheit,=20 which then causes the peroxide catalyst to decompose, thus = creating=20 what are known as free radicals.

"The free radical encounters = a molecule=20 of MMA and opens up the double bond," explains Minghetti. = "Suppose=20 you are a molecule of MMA and you have your hands crossed on = your=20 chest. The action of the free radical makes you open up your = arms to=20 the outside. The free radical links up to the left arm and = transfers=20 the active charge to the right arm. So the tip of your right = arm=20 becomes active or capable of opening the next double bond = when it=20 encounters another molecule of MMA. This is what is called a = chain=20 reaction, or a free radical polymerization. You have = thousands of=20 molecules that join in a chain, which is called a polymer=20 chain."

The energy released from all = these=20 thousands of chain reactions has the net effect of raising = the=20 temperature of the sheet. If that rise in temperature is not = carefully controlled it could result in a sheet full of = bubbles, or=20 even worse. The Aristech system uses heated water cascading = over the=20 outside of stainless steel belts to raise the temperature of = the=20 solid surface mixture sufficient to start the = polymerization. Later,=20 when the chain reaction is underway and the temperature = begins to=20 rise, the same water keeps the temperature from rising too=20 high.

"You have to remove most of = the heat of=20 polymerization so that the temperature of the sheet does not = exceed=20 the temperature of around 210 F," says Minghetti. "Most of = the=20 reaction is finished at that point and you have 85% to 90% = of the=20 job done. After that, you have to raise the temperature once = again=20 in the ovens to polymerize the remaining 10% to 15% residual = monomer."

Maintaining such precise = temperature=20 controls requires a massive casting machine that has water = pipes,=20 ovens and continuously moving stainless steel casting belts = all=20 built in. It is an expensive proposition, which requires an=20 extensive support system for storing and handling raw = materials and=20 finished goods as well. Thus, most acrylic-based solid = surface=20 production is limited to a handful of plants owned and = operated by=20 large, multi-national corporations scattered around the=20 world.

Yet, other methods for = casting acrylic=20 solid surface sheets do exist. SRS=EF=BF=BDs Shafran says = that pre-promoted,=20 acrylic reactive resins are available today designed with = the=20 medium-sized solid surface manufacturer in mind. "It is = similar to=20 the way that many unsaturated polyester resins are = supplied," he=20 explains. "The mindset of using acrylics is that you have to = blend=20 monomers together, or you have to make syrups or cook them. = The=20 pre-promoted products are formulated and ready to use as a = binder=20 resin."

What that means is a careful=20 manufacturer, experienced in producing solid surface using = polyester=20 resins, should be able to cast acrylic resin-based solid = surface by=20 implementing a few minor alterations to his existing = process.=20 "Obviously, for the shop that has been working traditionally = in=20 polyester," Schafran says, "this affords them the = opportunity to=20 offer their own high quality acrylic based product. It does = not=20 require a tremendous changeover or investment in equipment, = although=20 you do have to get used to the fact that you are running in = acrylic=20 now instead of polyester. It is a different = animal."

As is the case with the = larger acrylic=20 manufacturing processes, Schafran says that heat management = is a=20 critical issue when casting acrylic solid surface. Yet, an=20 experienced manufacturer, used to paying close attention to = details,=20 can be successful using the specially designed acrylic=20 resin.

Polyester

Polyester is a = petroleum-based product=20 that is formed from the building blocks of acids and glycol. = "You=20 can think of the acids and glycols as boys and girls," = explains=20 Louis Ross, Director of Research and Development for the = Thermoset=20 Resins Division of Interplastic Corporation. "When these = acids=20 (boys) and glycols (girls) react they form esters. If = you=20 think of the boys and girls holding hands, when they hook = together=20 you can have polyesters."

This relationship between = acids and=20 glycols is what forms the basis for both saturated and = unsaturated=20 polyester resins. Saturated polyester is = thermoformable,=20 which means that it can be heated and formed into new = shapes.=20 Fibers, film and injection molded plastic can all be made = from=20 saturated polyester resins.

Unsaturated polyester = resin=20 features molecule chains with acids and glycols hooked = together and=20 then cross-linked into rigid chains. Once unsaturated = polymers are=20 produced they are normally dissolved into monomers. The most = common=20 monomer used to dissolve, and ultimately crosslink the = polyester, is=20 styrene. However, most unsaturated polyester solid surface = resins=20 also contain methyl-methacrylate. After cross-linking = occurs, the=20 resin can no longer be melted and re-formed. This is why = polymers=20 that crosslink are known as thermoset.

As is the case with acrylic=20 polymerization, polyester relies on free radicals to = initiate and=20 maintain the chain reaction that turns the resin from a = liquid to a=20 solid. And, like acrylic resins, organic peroxide is the = initiator=20 that starts the polymerization.

"Chemically, the = peroxide is a=20 molecule that will break into at least two pieces," Ross = explains.=20 "That breakdown makes it active. These active fragments = react with=20 the unsaturation in the monomer or polyester to produce = another=20 active fragment. The active site continues to connect = monomer with=20 the polyester to form a crosslinked network. When one of = these=20 active or free radicals, runs into another one they = chemically react=20 and become inactive."

Once the chain reaction of = polymerization=20 is started it progresses rapidly until most of the styrene = monomer=20 and polyester molecules have bonded. The chemical reactions = give off=20 energy in the form of heat, creating what is known as an = exotherm.=20 At some point in the reaction the liquid mixture morphs into = a=20 rubbery gel state, where the molecules have grown so bulky = that they=20 find it hard to move within the mixture. Finally the whole = thing=20 continues its polymerization until it becomes rigid, or = enters what=20 is known as glass transition. But the curing process = doesn=EF=BF=BDt end=20 there.

A certain percentage of = styrene monomer=20 may become trapped within the rigid piece as it enters glass = transition. In order to ensure proper cure it is important = to keep=20 the composite at a temperature near the glass transition = temperature=20 long enough to finish the polymerization reactions. This = provides=20 freedom of motion to allow the reactions to occur. The best = time to=20 apply heat is right after the exotherm so that there is = sufficient=20 peroxide remaining to finish the cure. If complete cure is = not=20 attained with the exotherm, the piece must be re-heated = until it=20 reaches its rubbery state. Typically that is done in a = post-curing=20 oven. Thus, in a fully cured product there is no peroxide = left, and=20 the monomers have been all used up.

Fully cured thermoset = polyester exhibits=20 a high resistance to heat, which means it tends to expand = and=20 contract less as the temperature changes. And, if used with = the=20 right combination of fillers, polyester is also very = resistant to=20 stain.

The Choices are Practically=20 Endless

Both acrylic and polyester = resins can be=20 modified for specific applications, which generally results = in a=20 trade-off in performance characteristics. It is possible to = make a=20 somewhat thermoformable polyester solid surface, for = example, but it=20 would most likely have a problem with heat resistance. A = more highly=20 cross-linked acrylic resin, on the other hand, would be more = rigid=20 and non-thermoformable.

In the end analysis, the = total=20 performance package of any solid surface product will depend = on the=20 specific combination of resins, fillers, pigments and = catalysts=20 employed in its manufacture. But one thing is certain: a=20 manufacturer must pay strict attention to detail during the = entire=20 manufacturing process. To do otherwise would result in a = product=20 that does not perform up to=20 expectations.