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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The molecular forces and chemical bonding are important to understand the physical properties the bulk polymer exhibits. The covalent bonding in a polymer system is one of the strongest and significant forces and is often referred as primary bonding. A list of common covalent bonds and the energy associated with the bonds are listed below:

This list shows the dissociation (bond breaking) energy to predict which bonds will break first when a polymer is overheated or modified by plasma or UV. For example, the carbon (C) chlorine (Cl) bond will dissociate before a C-H bond in a PVC polymer.

Secondary bonding forces are also important as they can affect the material’s physical properties, such as surface tension, viscosity, friction, volatility, and solubility. The secondary forces are as follows:

Blends are the physical blend of polymers. Unlike a copolymer that is chemically mixed, polymer blends are mixed before or during molding operations. Yet, polymer blends can be just as useful and cost effective as copolymers or terpolymers. The physical property of a blend is determined by the physical properties of each ingredient and miscibility. If the compounds are miscible, the mixture will remain uniformly blended (homogenous). If the compounds are not miscible, the each mixture will separate into its respective phase. It would be here that the physical properties of the material would be compromised if the adhesion between the compounds is poor. Fortunately, if two immiscible polymer need to be blended, an additive can be used or a polymer can be grafted to one of the originals.

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A copolymer, though, is a polymer that has different numbers of repeating units. Copolymers are grouped according to the arrangement of the units in the polymer backbone.

The arrangement of the monomers in a copolymer is determined by the monomer types, the ratios between monomers, and processing conditions. Copolymers are the chemical mixture of two polymers in some ratio. Terpolymers are the chemical mixture of three different monomers like ABS. An engineer will polymerize monomers together to enhance strength, temperature resistance, or chemical resistance. Monomer forms can also be created through the parylene coating process.

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A polymer is a very large molecule (macromolecule) composed of many small repeating molecular units (monomer). Polymers are formed from atoms that are capable of multiple covalent bonds. Such as the carbon atoms in ethylene CH2=CH2 molecule. Molecules with this type of bonding are said to be unsaturated. These compounds tend to keep this structure yet will readily react (under heat and pressure) to form more stable single bond structures; they will form a saturated compound. For example, ethylene will react to form polyethylene [-CH2-CH2-’]n . The [n] signifies the number of repeating units in the polymer backbone. This number can be from 1000 to ~300,000 units. The polyethylene material will have different properties based on the number of repeating ethylene monomer units.

From this simple compound, substitutions can be made to provide different properties. When one substitution is made the compound is a vinyl monomer. When two substitutions are made the compound is a vinylidene monomer. As more substitutions are made other compounds are created.

To recap, polymers are formed through chemical reactions under heat and pressure. Additives, ingredients, and conditions are designed to control how the polymer is formed and desired properties. This process is called polymerization. Polymerizing one kind of monomer will create a homopolymer as in polyethylene or polypropylene.

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There are times when chemist / engineers need a little help remembering how to make a really cold bath…so here it is…

Blocking - The greater the level of treatment, the higher the degree of oxidation of the surface. The polar groups formed by the corona have an attraction for the molecular layer on the other side of the web, and when the two sides come into contact when they are on the roll, a self-adhering condition exists.  Sometimes this attraction can be greater than the internal bonds of the substrate so that delamination of the substrate can occur when the product is unrolled.  The tighter the roll is wound and the longer it is in storage the more severe the problem becomes.  Blocking is worse in the film at the center of the roll.

Heat Sealing – Excessive treatment also leads to problems when attempting to heat seal the product.

Additives – If the polypropylene or polyethylene contain additional components, such as slip additives or some processing aids, the initial treatment is reduced over time as these additives bloom to the surface and partially mask the polar groups formed during treatment. For this reason, it is better to treat these films at the point of use rather than the point of manufacture.

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This study examined the relative adhesion difference between untreated polycarbonate (PC), mechanically roughened PC, and plasma treated PC. It appears that plasma surface modification of PC based polymers is a viable way to enhance adhesion prior to bond-up, lamination, or overmolding. This study observed approximately a 459% increase in lap shear bond strength after plasma treatment.

 Polycarbonate is a specific group of thermoplastics. They are called polycarbonates because they are polymers having functional groups linked together by carbonate groups in a long molecular chain.

The most common type of polycarbonate plastic is one made from Bisphenol A, in which groups from Bisphenol A are linked together by carbonate groups in a polymer chain. This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass. Polycarbonate can be mechanically bonded by standard methods. It can also be cemented by using a solvent such as methylene chloride or adhesives such as epoxy, urethane and silicone. Polycarbonate and also be ultrasonically welded. Yet, solvent based adhesive can contaminate sensitive devices. Moreover, ultrasonic welding requires tight tolerances and smooth contaminate-free surfaces. The plasma treatment prior to bonding with common adhesive has shown an effective way to bond PC without solvent based adhesive or technically difficult, sonic welding. 

A plasma is a quasineutral cloud of ion, electrons, and radicals. The diffuse cloud is capable of doing chemistry on the surface of materials that is unique, providing wettable or adherent surfaces on materials that are otherwise inert.

The PC samples in this study were subjected to a specific plasma gas mixture to induce and adherent surface for a structural epoxy adhesive. The results are as follows:

In summary, polycarbonate can be bonded using mechanical or solvent chemical methods. Yet, it has been proven that plasma surface modification is a viable, environmentally friendly, invisible treatment that can enhance the bonding performance significantly. If you would like more information about this process or other processes, please contact us at www.tstar.com.

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!

Polyethylene
A family of thermoplastic resins obtained by polymerizing the gas ethylene [C₂H₄]. Low molecular weight polymers of ethylene are fluids used as lubricants; medium weight polymers are waxes miscible with paraffin; and the high molecular weight polymers (i.e., over 6000) are the materials used in the plastics industry. Polymers with densities ranging from about .910 to .925 are called low density; those of densities from .926 to .940 are called medium density; and those from .941 to .965 and over are called high density. The low density types are polymerized at very high pressures and temperatures, and the high density types at relatively low temperatures and pressures. A relatively new type called linear low density polyethylene is manufactured through a variety of processes: gas phase, solution, slurry, or high pressure conversion. A high efficiency catalyst system aids in the polymerization of ethylene and allows for lower temperatures and pressures than those required in making conventional low density polyethylene. Copolymers of ethylene with vinyl acetate, ethyl acrylate, and acrylic acid are commercially important. Major polyethylene applications can be found in packaging, housewares, toys and communications equipment. Can be bonded effectively after plasma treatments or by using our UltraFlon Bond-X 1606.

Polyimides
A family of thermoset and thermoplastic resins characterized by repeating imide linkages: There are four types of aromatic polyimides: (1) condensation products made by the reaction pyromellitic dianhydride (PMDA) [C₆H₂(C₂O₃)₂] and aromatic diamines such as 4,4′-diaminodiphenyl ether [(C₆H₄NH₂)₂O]; (2) condensation products of 3,4,3′,4′-benzophenone tetracarboxylic dianhydride (BTDA) [(C₆H₅)₂CO(C₂O₃)₂] and aromatic amines;(3) the reaction of BTDA and a diisocyanate such as 4,4′-methylene-bis(phenylisocyanate) [OCNC₆H₄CH₂C₆H₄NCO]; and (4) a polyimide based on diaminophenylindane and a dicarboxylic anhydride such as carbonyldiphthalic anhydride [OC₆H₄(CO)₂COC₆H₄(CO)₂]. Thermoset polyimides are produced in condensation polymers that possess reactive terminal groups capable of subsequent cross-linking through an addition reaction. Typical applications for thermoplastic and thermosetting polyimides are transportation and electronics. Can be bonded after plasma treatment using most epoxies.

Polyphenylene Oxide, Modified
Engineering thermoplastic resins produced by the oxidative coupling of 2, 6-dimethylphenol [(CH₃)₂C₆H₃OH] (xylenol), then blended with impact polystyrene. Typical applications are found in electrical/electronic uses, business machine parts, appliances, and automotive parts. Can be bonded with solvents or epoxies. Adhesion can be enhanced greatly after plasma treatments.

Polyphenylene Sulfide
Engineering thermoplastic resins produced by the reaction of p-dichlorobenzene [C₆H₄CI₂] with sodium sulfide [Na₂S]. The major use for polyphenylene sulfide is in electrical/ electronic parts and automotive parts. After plasma treatments, this material bonds effectively with most epoxies.

Polypropylene
Thermoplastic resins made by polymerizing propylene [CH₃CHCH₂] and in the case of copolymers with monomers, with suitable catalysts, generally aluminum alkyl and titanium tetrachloride mixed with solvents. The monomer unit in polypropylene is asymmetric and can assume two regular geometric arrangements: isotactic, with all methyl groups aligned on the same side of the chain, or syndiotactic, with the methyl groups alternating. All other forms, where this positioning is random, are called atactic. Commercial polypropylene contains 90-97% crystalline or isotactic PP with the remainder being atactic. Most processes remove excess atactic PP. This by-product is used in adhesives, caulks, and cablefilling compounds. Major applications of commercial PP are found in packaging, automotive, appliance and carpeting markets. This material can be bonded effectively using UtraFlon Bond-X 1606.

Polystyrene
High molecular weight thermoplastic resins produced generally by the free-radical polymerization of styrene monomer [C₆H₅CHCH₂] which can be initiated by heating alone but more effectively by heating in the presence of free-radical initiator (such as benzoyl peroxide [(C₆H₅CO)₂O₂]. Typical processing techniques are modified mass polymerization or solution polymerization, suspension polymerization, and expandable beads. Major markets for polystyrene are in consumer and institutional products, electrical/electronic uses, and building/ construction. Typically there are no issues bonding this material. But, plasma treatments have been used to enhance wettability of the material.

Polyurethanes
A large family of polymers based on the reaction product of an organic isocyanate with compounds containing a hydroxyl group. The commonly used isocyanates are toluene diisocyanate (TDI) [CH₃C₆H₃(NCO)₂], methylene diphenyl isocyanate (MDI) [OCNC₆H₄CH₂C₆H₄NCO], and polymeric isocyanates (PMDI), obtained by the phosgenation of polyamines derived from the condensation of aniline [C₆H₅NH₂] with formaldehyde (HCHO]. Polyols (with hydroxyl groups) are macroglycols which are either polyester or polyether based. Polyurethane elastomers and resins take the form of liquid castings systems thermoplastic elastomers and resins, microcellular products, and millible gums. Typical applications are found in the automotive industry. Polyurethane foams are widely used in transportation, furniture, and construction markets. Can be bonded effectively using acrylic adhesive or urethane adhesive. Adhesion can be improved greatly after plasma treatment.

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