Tech Talk Blog

The dynamics of wetting are described below:

Spreading = A – ( B+ C )

Where:

• A = surface energy of solid (given below)
• B = surface tension of liquid
• C = surface energy of solid-liquid interface

If Spreading is:

• Negative. Then, liquid will bead up.
• Zero. Then, liquid will spread.
• Positive. Then, liquid will spread.

If the material surface energy is relatively low, then the coating will not flow well and fisheyes, pinholes, gaps, or air bubbles will form. If the material surface energy is too high, then the paint, ink, or coating may bleed or be difficult to control. Therefore, the surface tension of the liquid and the surface energy of the material must be matched for the application.

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There are many ways in which polymer properties or behaviors are classified to make general descriptions and understanding easier. Some common classifications are:

• Thermoplastic / thermoset
• Amorphous / crystalline
• Addition / condensation

Thermoplastic vs. Thermoset

Thermoplastics are materials which can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point.

Thermoset materials undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network. Once they are heated and formed they can not be reprocessed – the three-dimensional molecules can not be made to flow under pressure when heated.

Amorphous vs Crystalline Polymers

Crystalline polymers are polymers with nearly linear structure, which tends to be flexible and fold up to form tightly, packed and ordered “crystalline” areas. Time and temperature during processing influence the degree of crystallinity. Crystalline polymers include: polyethylene, polypropylene, acetals, nylons, and most thermoplastic polyesters. Crystalline polymers have higher shrinkage, are generally opaque or translucent, with good to excellent chemical resistance, low surface friction, and good to excellent wear resistance.

Amorphous polymers are polymers with bulkier molecular chains or large branches or functional groups, which tend to be stiffer and will not fold up tight enough to form crystals. Common amorphous polymers include polystyrene, polycarbonate, acrylic, ABS, SAN, and polysulfone. Amorphous polymers have low shrinkage, good transparency, gradual softening when heated (no distinct melting point), average to poor chemical resistance, high surface friction, and average to low wear resistance.

Condensation vs. Addition Polymers

Condensation polymers such as nylons, acetals, and polyesters are made by condensation or step-reaction polymerization, where small molecules (monomers) of two different chemicals combine to form chains of alternating chemical groups. The length of molecules is determined by the number of active chain ends available to react with more monomer or the active ends of other molecules.

Addition polymers such as polyethylene, polystyrene, acrylic, and polyvinyl chloride are made by addition or chain-reaction polymerization where only one monomer species is used. The reaction is begun by an initiator which activates monomer molecules by the breaking a double bond between atoms and creating two bonding sites. These sites quickly react with sites on other monomer or polymer molecules. The process continues until the initiator is used up and the reaction stops. The length of molecules is determined by the number of monomer molecules which can attach to a chain before the initiator is consumed and all molecules with initiated bonding sites have reacted.

In summary, a polymer is a very large molecule made up of repeating small molecular groups. The elements and bonds of a polymer give the polymer its bulk properties. All too often a polymer will be designed for the easy of molding or processing and not for subsequent processes like bonding, painting, printing, or coating.

It is at this point where surface modification of the polymer is essential. The polymer can be treated after the polymer has been molded, extruded, formed, coated, or cast without changing the bulk properties of the polymer. So, an engineer can specify a material that would best suit high volume manufacturing and device integrity without compromising the device due to a printing of painting process.

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Polyvinyl Acetate (PVAc) & Other Vinyls
Polyvinyl acetate is a thermoplastic resin produced by the polymerization of vinyl acetate monomer [CH₃COOCHCH₂] in water producing an emulsion with a solids content of 50-55%. Most polyvinyl acetate emulsions contain co-monomers such as n-butyl acrylate, 2-ethyl hexyl acrylate, ethylene, dibutyl maleate and dibutyl fumarate. Polymerization of vinyl acetate with ethylene also can be used to produce solid vinyl acetate/ethylene copolymers with more than 50% vinyl acetate content. Polyvinyl alcohol (PVOH) is produced by methanolysis or hydrolysis of polyvinyl acetates. The reaction can be controlled to produce any degree of replacement of acetate groups. Co-polymers of replaced acetate groupings and other monomers such as ethylene and acrylate esters are commercially important. Polyvinyl butyral (PVB) is made by reacting PVOH with butyraldehyde [CH₃(CH₂)₂CHO]. Polyvinyl formal is made by condensing formaldehyde [HCHO] in presence of PVOH or by the simultaneous hydrolysis and acetylization of PVAc. Polyvinylidene chloride is made by the polymerization of 1,1-dichloroethylene [CH₂CCL₂]. Typical applications for the above resins are found in adhesives, paints, coatings and finishes, and packaging.

Polyvinyl Chloride
Thermoplastic resins produced by the polymerization of the gas vinyl chloride [CH₂CHCl]. Under pressure, vinyl chloride becomes liquefied and is polymerized by one of four basic processes: suspension, emulsion, bulk, or solution polymerization. The pure polymer is hard, brittle and difficult to process, but it becomes flexible when plasticizers are added. A special class of PVC resin of fine particle size, often called dispersion grade resin, can be dispersed in liquid plasticizers to form plastisols. The addition of a volatile diluent or a solvent to the plastisol produces an organosol. Copolymers with vinyl acetate, vinylidene chloride, and maleate and fumarate esters find commercial application. Major markets for PVC are in building/construction, packaging, consumer and institutional products, and electrical/electronic uses. This material bonds effectively using solvents. Plasma treatments can enhance the adhesion of this material if solvents are not used.

Styrene Acrylonitrile
Thermoplastic copolymers of styrene [C₆H₅CHCH₂] and acrylonitrile [CH₂CHCN]. SAN resins are random, amorphous copolymers produced by emulsion, suspension, or continuous mass polymerization. Typical uses include automobile instrument lenses and housewares. Typically, this material does not have adhesion issues.

Styrene Butadiene Latexes & Other Styrene Copolymers
Styrene butadiene latexes usually have a resin content of about 50%. The styrene/butadiene ratio varies from 54:46 to 80:20. Most are carboxylated by the use of such acids as maleic [HOOCCHCHCOO], fumaric [HOOCCHCHCOOH], acrylic [CH₂CHCOOH], or methacrylic [CH₂C(CH₃)COOH]. Two types of styrene-maleic anhydride (SMA) [(COCH)₂O] are available: SMA copolymers, with and without rubber impact modifier (e.g., DYLARK¨) and SMA terpolymer alloys (e.g., CADON¨). K-Resin¨ is a solid styrenebutadiene copolymer resin. Acrylic monomers are also used in conjunction with styrene (or styrene plus other monomers) to produce specialty resins. For example, there are transparent terpolymers of methyl methacrylate, butadiene, and styrene (MBS), and others of acrylonitrile, an acrylic monomer, and styrene (AAS). Ion-exchange resins or divinylbenzene-modified polystyrene are another variation. SB latexes are used in carpet backing and paper coatings. The other styrenics are used in paints, coatings, and floor polishes, plus many other uses. Typically this material can be bonded using solvents. Moreover, these materials are enhanced after plasma treatment using other adhesives.

Sulfone Polymers
A family of engineering thermoplastic resins characterized by the sulfone [SO₂] group. Polysulfone is made by the reaction of the disodium salt of bisphenol A[(CH₃)₂C(C₆H₄OH)₂] with 4,4′- dichlorodiphenyl sulfone 4,4′-DCDPS [(C₆H₄Cl)₂SO₂]. Polyethersulfone is made by the reaction of 4,4′-DCDPS with potassium hydroxide [KOH]. Polyphenylsulfone is similar to the other sulfone polymers. Typical applications for sulfone polymers are found in electrical/electronic uses and automotive parts. Plasma treatments often enhance the adhesion of this material significantly using epoxies.

Thermoplastic Polyester (Saturated)
A family of polyesters in which the polyester backbones are saturated and hence nonreactive. The most common commercial types are: PET (polyethylene terephthalate) produced by polycondensation of ethylene glycol [CH₂OHCH₂OH] with either dimethyl terephthalate (DMT) [C₆H₄(COOCH₃)₂] or terephthalic acid (TPA) [C₆H₄(COOH)₂]; and PBT (polybutylene terephthalate) produced by the reaction of DMT with 1,4 butanediol [HO(CH₂)₄OH]. Typical applications are found in packaging, automotive, electrical, and consumer markets. Plasma treatments enhance this material when using epoxy.

Unsaturated Polyester
Thermosetting resins made by the condensation reaction between difunctional acids and glycols. The resulting polymer is then dissolved in styrene [C₆H₅CHCH₂] or other vinyl unsaturated monomer. The structures of the acids and glycols used and their proportions, especially the ratio of the unsaturated versus the saturated acid, and the type and amount of monomer used, are all tailored for each resin to balance economy, processing characteristics, and performance properties. One common formulation is the reaction of maleic anhydride [(COCH)₂O], phthalic anhydride [C₆H₄(CO)₂O], and propylene glycol [CH₃CHOHCH₂OH]. Both dicyclopentadiene [C10H1₂] and isophthalic acid [C₆H₄(COOH)₂] can be substituted for phthalic anhydride. Vinyl ester resins are linear reaction products of bisphenol A [(CH₃)₂C(C₆H₄OH)₂] and epichlorohydrin [CH₂OCHCH₂Cl] that are terminated with an unsaturated acid such as methacrylic acid [CH₂C(CH₃)COOH]. Typical applications are found in transportation, appliances, electrical, and construction markets. As in the above material, plasma treatments enhance this material when using epoxy.

Urea-Formaldehyde
Formed by the condensation reaction of formaldehyde [HCHO] and urea [CO(NH₂)₂]. These thermoset resins are clear water-white syrups or white powered materials which can be dispersed in water to form colorless syrups. They cure at elevated temperatures with appropriate catalysts. Molding powders are made by adding fillers to the uncured syrups, forming a consistency suitable for compression and transfer molding. The liquid and dried resins find extensive uses in laminates and chemically resistant coatings. The molding compounds are formed into rigid electrical and decorative products.

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Hope those were enough definitions for you!

Gamma Radiation is commonly used to sterilize polymer devices. But some polymers do not hold up well to this form of sterilization.

The following plastics cannot be sterilized by radiation:

• Polyacetals (turns to dust)
• Polyacetals(turns to dust)
• Polypropylene (unstabilized)
• Teflon (turns to wax)
• PVDF

Polypropylene Syndrome

• Natural unstabilized PP undergoes a slow degradation process after irradiation where over two years the elongation may drop from 600% to zero and parts will shatter
• PP is both crosslinked and scissioned
• Embrittlement and discoloration can occur
• Radiation stable PP is available

 Polyethylene

• Polyethylene is predominantly crosslinked but acceptable to irradiation
• LDPE < LLDPE < HDPE < UHMWHDPE
• PE can be stabilized to make it gamma stable

If you are unsure if your material will hold up to this sterilization technique, please visit the TriStar Plastics Corp. website for contact information.