Coatings of Polymers and Plastics Part 7 pps

Silane func-tionality also can provide a high crosslink density due to the multiple number ofalkoxy groups attached, all of which can participate in crosslinking reactions ifthe conditio

3.1.6 Silane

There are a number of ways to introduce organosilane functionality onto mer backbones Silane functionality for coatings exists in the form shown in thefollowing text

poly-R1Si(OR)xR1= polymer backbone R = CH3or other small alkyl

In acrylic resins, this is accomplished by copolymerizing an acrylate or acrylate containing organosilane The most readily available of these isγ-trimethoxysilylpropyl methacrylate (TMSPM) The advantages of silane func-

meth-tionality is the high degree of flexibility and hydrolytic stability in the Si−O−C

or Si−O−Si bonds that are formed in the curing reaction The curing reactioncan be initiated by moisture or by reactions with hydroxyl functional crosslink-ers as shown in Figure 14 (22,23) In moisture cure reactions, the alkoxy silane

functional group reacts first with water to form a silanol (SiOH) group Thisoccurs both with acid and base catalysis The silanol group then self-reacts toform a siloxane bond (Si−O−Si) The siloxane bond provides a great deal offlexibility to a crosslink structure and is very resistant to hydrolysis Silane func-tionality also can provide a high crosslink density due to the multiple number ofalkoxy groups attached, all of which can participate in crosslinking reactions ifthe conditions are rigorous enough (catalysis, availability of moisture, cure tem-perature) The disadvantages of organosilane functionality are high cost and soft-ness (low Tg) that is inherent when the very flexible siloxane bonds are present.

3.2 Polyester Binders

High-performance coatings for plastic substrates are often formulated with ester resins that are then crosslinked with materials similar to those describedfor acrylic resins Polyesters are prepared by a step growth polymerizationmechanism from polycarboxylic acids (or their anhydrides) and polyols Just as

poly-in acrylic bpoly-inders, a wide variety of properties can be formulated by the choice

of the polyacids and polyols As a class, polyesters are often not thought of as

a super-durable building block, but polyesters can be very durable Polyestersalso are more easily designed to have better flexiblity and impact resistance thanacrylic resins In acrylic resins, the polymer backbone is always carbon-to-carbonbonding with some bulky substituent on that backbone This configuration re-stricts the ability of that polymer molecule to rotate and yield more flexiblematerials In polyesters, the polymer backbone that has a series of ester linkagescan be designed to have a significant amount of rotational movement and pro-vide materials with greater flexibility The carbon-carbon backbone bonding of

an acrylic resin is not as susceptible to chemical degradation reactions as apolyester backbone that consists of a string of ester linkages, which are capable

of being degraded by chemical attack

Figure 15 shows a number of commonly used polyols and polycarboxylicacids used in making polyesters for coating purposes (In polyesterifications,anhydrides behave like difunctional acids.) Like the acrylic monomers, the eco-nomics of these building blocks are often associated with the production volume

of the material Often it is the noncoating uses of the monomeric materials thatgovern this cost The aromatic acids (phthalic and isophthalic) provide rigidlinkages with properties of hardness and stiffness The aliphatic acids like adipic

or azelaic, contribute sequences of methylene (CH2) linkages that provide bility As with acrylic monomeric materials, the aromatic containing buildingblocks can absorb some of the wavelengths of sunlight (and even more of thewavelengths found in accelerated weathering testers) making the polyester moresusceptible to photooxidative degradation reactions and therefore less durable.For this reason, aliphatic polyesters are more widely used by today’s coatingsfor plastic formulators

flexi-F IG 15 Some commonly used polyols and polycarboxylic acids used in makingpolyesters for coating purposes.

The utilization of polyacids or polyols that place cycloaliphatic rings alongthe polyester backbone is thought to provide a better balance of hardness andflexibility or impact resistance than is attainable otherwise (24) Examples ofthis type of monomer is cyclohexanedimethanol (CHDM) and 1,4 cyclohexanedi-carboxylic acid (CHDA) This property may be due to the ability of the cyclo-hexane ring to change conformations (chair to boat) as a mechanism for absorb-ing energy without causing bond breakage

Branching of polyesters is accomplished by using polyols with ity greater than two (e.g., trimethylol propane [TMP] or pentaerythritol [PE]).Branching of polymeric materials is another mechanism for introducing the po-

functional-tential for flexibility and impact resistance The branches can prevent somepolymer chain/polymer chain interactions that may cause stiffening Branchingmay provide higher crosslink density to thermosetting polyesters This is animportant property for mar and scratch resistance and for resistance to attack bychemical agents and moisture The amount of monomeric building blocks thathave functionality greater than two is limited in the step growth polymerizationprocess for preparing polyesters When the average functionality of the mono-mers is too high, the polyester will gel in preparation Discussions of this phe-nomena is found in polymer textbooks (25).

3.3 Polyurethane Binders

Polyurethanes are prepared in step-growth polymerization processes similar tothat used for preparing polyesters In fact, many polyurethane materials are hy-brids of ester linkages and urethane linkages If a polyisocyanate is substitutedfor the polycarboxylic acid shown in Figure 2, the result is a polyurethane Theurethane linkage is obtained by the reaction of an alcohol with an isocyanate.(The urethane linkage is also known as a secondary carbamate.)

R′−OH

alcohol+R−N=C=O

urethaneFigure 16 illustrates polyisocyanates that are commonly used to preparepolyurethanes The aromatic polyisocyanates are significantly less expensivethan the aliphatic types Again, this is due to the larger production volumes of

the aromatic materials that find use in foams and other structural applications.Aromatic urethanes not only suffer from poorer durability than the aliphatictypes, similar to aromatic polyesters and styrene-containing acrylics, but theyare known to yellow severely upon exposure to sunlight In coatings, aromaticurethane binders are limited to use as primers and undercoats Even in theseapplications, a user must be careful to protect these undercoats from exposure

to UV light either with sufficient hiding pigmentation or with UV-absorbingadditives

Urethane linkages in coatings provide toughness and flexibility to thebinders As discussed in the Environmental Etch Resistance section (Sec 3.1.3),they are also more resistant to hydrolytic events than ester linkages and canprovide better chemical-resistance properties The toughness property is ex-plained by the formation of interpolymer hydrogen bonding in the coating Thehydrogen bond between the imino (NH) and the carbonyl (C= O) can be brokenunder the stress of impact, absorbing energy, and then reforming after the stressevent (26) Figure 17 demonstrates this type of hydrogen bonding The polarnature of the urethane bond also accounts for chemical resistance propertiesversus oily material exposure Historically, alkyd coatings have been modified

by reactions with polyisocyanates to give them better resistance to gasoline andoils

An important class of polyurethanes are waterborne polyurethane sions (PUD) (27–29) The PUD has been a component of “soft touch” coatingsand finds significant usage in other waterborne coatings as a component thatadds toughness and cohesive strength to the coating The difference betweenPUDs and other polyurethane resins described above is that ionizable groups(usually carboxyl) are incorporated on the polyurethane backbone This allowsthe polymer to be dispersed in water after neutralization of those carboxylgroups by amines As previously discussed, the properties of the PUD can bevaried substantially depending on the structure of the polyol and diisocyanatematerials used to prepare them Further discussion of the ability to be handled

in water and the tradeoffs that result by making polymers useable in aqueouscoatings will be described in the Polymers for Waterborne Coatings Section(Sec 5.2).

3.4 Polyether Modifications of Polyesters and Polyurethanes

In order to accommodate the need for flexibility (or lower cost) in coatings forsome plastic substrates, polyols with internal ether linkages can be utilized Inthis case, a carbon-oxygen-carbon (COC) linkage replaces a CCC linkage TheCOC bond is significantly more flexible than the CCC bond Series of polyetherpolyols are easily prepared from ethylene oxide and propylene oxide (and othercyclic ethers), which makes them very economical and provides a large number

of materials for dialing in a desired balance of hardness and flexibility Again,the low cost of these materials is from their high production volumes due totheir use in plastic materials Figure 18 shows the synthesis of polyether polyols.The disadvantage of polyethers is their poor photooxidative durability Acarbon-hydrogen bond adjacent to the ether linkage is attacked by free radicalsources to yield peroxides that subsequently cause backbone scission and/or lead

to moieties in the coating (like carbonyl groups) that can cause more degradation

or color As with the aromatic urethane linkages, polyether linkages are oftenonly used in primers or other applications where photooxidative durability isnot a primary property

The polyethers of ethylene oxide (ethoxylates) can be used to providehydrophilicity to a polymer backbone and consequently water solubility or dis-persibility This is a desirable property if waterborne coatings are required, butdoes lead to some degree of water or humidity sensitivity of the coating

with many functional groups and lower molecular weight are considered thecrosslinkers (versus the primary binder) Here, we will discuss two materials ofthis type—polyisocyanates and amino resins They are used to react with acrylicresins, polyester resins, and other polymeric backbones containing active hydro-gens, and form useful coatings.

3.5.1 Polyisocyanates

Polyisocyanates are versatile crosslinkers for plastics coatings with favorablefeatures of low-temperature cure, chemical resistant bonding, and flexibility.Polyisocyanates are more expensive than amino resin crosslinkers, often do notprovide good mar-and-scratch resistance, and need very careful handling due tohygiene concerns Aliphatic polyisocyanates yield very durable, non-yellowingcoatings when formulated correctly Coatings crosslinked with aromatic polyiso-cyanates, which are more available and less expensive, will have poor durabilityand yellowing if exposed to sunlight Commonly used polyisocyanates areshown in Figure 19

Polyisocyanates are reactive with binders that contain active hydrogens(hydroxyl and amino), are self-reactive, and will react with ambient moisture togive cured coatings The most common reactions for curing coatings are shown

in Figure 20

Because of the reactive nature of isocyanate groups with the mentionedfunctional groups, polyisocyanates can be formulated to react under a wide vari-ety of cure conditions Many substances catalyze these reactions Common cata-lysts are organotin compounds, other metallic salts, amines, and acids Thisbreadth of catalytic species can also introduce problems, because catalysts may

be unknowingly introduced from other components of a formulation or even ascontaminants of other formulating materials

F IG 20 Cure reactions with isocyanates.

The reactivity of polyisocyanates, described previously, often requires thatthe coating system be provided in multicomponents that are mixed just prior touse It will also require protection of the isocyanate component from moistureduring storage and handling As a result, many coatings utilizing polyisocyanatecrosslinkers are known as “2K” coatings (The “K” comes from the German

“komponent.”) Because the components, after mixing, will react under ambientconditions, there will be a limited time that the mixture is usable The usablelifetime, which may vary depending on the definition of usablility, is known asthe “pot life.” After exceeding the pot life of the mixture, it may be too viscousfor application and performance properties of the coating will not match up tofresh mixes—or both

Along with the polyisocyanates that are commercially available, nate containing prepolymers can be prepared to provide even wider formulatingand handling latitude These prepolymers are often based on monomeric or poly-

isocya-meric polyols that have been reacted with diisocyanates to provide terminal,reactive crosslinking sites (Fig 21).

For durable polyurethane coatings, HDI trimer and IPDI trimer are mostcommonly used The trimers are formed by controlled reaction of the diisocya-nates The trimerization process yields higher viscosity, but introduces function-ality greater than two, that is necessary to form good crosslinked films Thetrimerization also removes volatile isocyanates and yields a material which can

be handled with practical protection schemes (As with any chemical substance,

a user must understand the personal protection necessary for handling.) Thistrimerization process will also generate higher molecular weight oligomers thatcan affect the performance properties and the application solids (and relatedsolvent emissions) At some higher cost, the manufacturers of these materialscan remove these higher molecular weight components Under different condi-tions the diisocyanates can be dimerized (called uretdiones), producing coatingswith a lower viscosity and a lower degree of crosslinking Mixtures of dimersand trimers are available for higher solids coatings

HDI trimer will provide a softer, more flexible coating than IPDI trimer.This is due to the structure that has a six carbon linear moiety separating thecenter of the trimer with the reactive isocyanate group The IPDI structure intro-duces a more rigid cycloaliphatic ring that yields a harder coating with lessability to flex The isocyanate associated with IPDI is somewhat slower reactingthan that associated with HDI and may require higher curing temperatures ormore catalyst Because the IPDI structure introduces inherent hardness, it isnecessary to check a coating that is thought to be cured for some other measure

of cure than hardness or dry time

3.5.2 Amino Resin Crosslinkers

Amino resins are formed from amino-containing (NH2) chemicals where theamino group is adjacent to an electron-withdrawing portion of a molecule Inthis configuration, the amino groups are not nearly as basic as amines Theseamino groups are reacted with formaldehyde to yield reactive methylol groups(Fig 22) The most commonly used precursors for amino resin crosslinkers areurea (22a) and melamine (22b) When reacted with formaldehyde they producepolyfunctional reactants that have three to six (or more) reactive sites on a fairlysmall molecule The methylol functional crosslinkers are further modified bythe reaction with alcohols to generate alkoxymethyl derivatives (22c) This mod-ification renders the amino resins more storage stable (both with themselves and

in formulations) and more soluble and compatible with the polymeric materialsthat they will be formulated with in a coating The primary reactions shown inthis section are accompanied by side reactions involving the condensation of theamino crosslinkers with themselves and the degree to which any of the reactionstake place can be controlled by the conditions of the reactions and the moleratios of the reactants This results in families of amino resin crosslinkers withranges of reactivity, solubility, and stability

The amino resins can react with several functionalities on the “mainbinder resins” such as hydroxyl, amide, carboxyl, and urethane (or carbamate).Most commonly, the binder resins are hydroxyl functional acrylics, polyesters,

or modifications of these Recently, there has been an increase in product ings and literature references to urethane or carbamate functional resins beingpaired with amino resins (16–18) The curing reactions require acid catalysisand heat There is a significant variety of reactivities (and therefore curing tem-peratures) possible with amino resins, but the lower temperature cure materials

are accompanied by poorer coating stability The reactions can be forced withvery high levels of acid catalyst, but then the coatings are likely to be verysensitive to moisture and have poor durability because the incorporated acidcatalyst can accelerate degradation reactions Typical cure conditions for aminoresin–cured coatings are in the range of 90°C to 150°C (200°F to 300°F).3.5.3 Other Crosslinking Strategies

Aside from the use of polyisocyanates and amino resins, other crosslinkingchemistries are being used for coatings for plastics Some of these (epoxy/acid,carbamate, and silane) were discussed in context of acrylic binders (Sec 3.1),but these crosslinking mechanisms can also be utilized with polyester bindersand polyurethane binders by incorporating appropriate functional groups onthose materials In this way, it is possible to gain the more flexible binder back-bones and achieve the advantages of the properties of other crosslinking chemis-tries (16,30)

4 RADIATION CURED COATINGS

Radiation (ultraviolet light and electron beam) is a means of curing coatingsand can be effectively used for heat-sensitive substrates because the curing can

be carried out at ambient temperatures This is accomplished by incorporatingfunctional groups into the binder resins that are activated by the radiation Thetwo commonly used functional groups are acrylate(methacrylate) functionalityand epoxy functionality These functionalities are shown in Figure 23

When the unsaturated acrylate functionality is utilized, curing is initiated

by free radicals that are caused by the interaction of the radiation with the bindermaterials or activators that are incorporated into the formulations This type of

coating can be formulated to very high solids by utilizing reactive acrylate omers or oligomers as diluents The curing mechanism proceeds by the chaingrowth polymerization mechanism (see Fig 1) As a result of utilizing highlyfunctional monomers or oligomers in the curing process, coatings with highcrosslink density can be generated High crosslink density often yields coatingswith very good mar and scratch resistance.

mon-This strategy may cause problems due to the potential toxicity of the late functional monomers or oligomers and curing is only effective in the line

acry-of sight acry-of the radiation source Free radical–initiated chain growth tion is also inhibited by oxygen, so inert atmospheres are necessary or formulascan be modified with materials that will react with oxygen This limits the use

polymeriza-of UV cure on complex parts This concern is being addressed by incorporatingdual cure mechanisms that initiate curing by the radiation and continue curingwith some thermally activated curing chemistry (31) Because it is relativelyeasy to modify many types of materials with the acrylate functionality, develop-ing a binder system with desirable properties to match the substrate and theproduct use is feasible

Epoxy chemistry is activated by the incorporation of initiators that ate “superacids” when exposed to UV radiation These acidic catalysts then startthe ring opening polymerization of the epoxy groups Epoxy groups that aresensitive to this type of curing are cycloaliphatic epoxies, as shown in Figure

gener-23 The glycidyl-type epoxy groups that were described previously in the acrylicbinder section are significantly less reactive toward this mechanism The modifi-cation of binder systems to cure through radiation-initiated epoxy polymeriza-tion is not as versatile, so structure/property development may be more limitedthan with the acrylate functional schemes

5 POLYMER REQUIREMENTS FOR HIGH SOLIDS,

WATERBORNE, AND POWDER COATINGS

In this section, the requirements to allow polymeric binders to function well inlow-emission (high solids, waterborne, and powder) coatings are discussed Therequirements will apply to all of the binder types (including crosslinkers) thatwere discussed in the previous sections Along with the features that allow thebinders to perform in the low-emission coatings, the limitations that are imposed

on the coatings are also discussed

5.1 High Solids (Low Solvent)

Solvents provide low viscosity to coatings that are needed for conventional plication methods (spray, roller coating, etc.) They also control the flow and