Acrylic Resin Fundamentals

Acrylic Resin Fundamentals, UL Prospector, Ronald Lewarchik, 4/2016:

Coatings utilizing acrylic resins are the leading polymer technology in the coatings industry. Historically alkyd finishes have held the leading position in coatings for decades. Acrylics are utilized in architectural coatings, product finishes for original equipment manufacture including automotive (OEM) and refinish, as well as special-purpose coatings.

Acrylic resins are primarily based on acrylate and methacrylate monomers and provide good weather resistance, resistance to hydrolysis, gloss and color retention in exterior applications. Due to their versatility and performance, acrylic coatings account for over 25% of all coatings and global sales approaching $25 billion. Acrylic resins can be thermoplastic or thermosett and are used in organic solvent born, waterborne, powder and radiation-curable coatings

Table I – Tg of Nonfunctional Homopolymers

Three broad classes of liquid coatings utilizing acrylic resins include thermoplastic, thermoset and waterborne. Many acrylic resins may also include other vinyl monomers such as styrene or vinyl acetate primarily to reduce cost. Acrylic monomers have a lower Tg than their analogous methacrylate monomers (for example compare the Tg for n-butyl acrylate versus n-butyl methacrylate see Table I and Table II). As Table II suggests, the glass transition temperature of the monomers selected for synthesis of a resin can be selected to enhance multiple properties that may include weather resistance, moisture resistance, oxygen permeability, flexibility reactivity, cure and hardness. In addition, acrylics can be functionalized with a variety of monomers to provide improved adhesion to metal, or to react for example with aminoplast or isocyanate crosslinkers.

acrylics_table_2 — Table II Relationship of Tg to Physical Properties

Thermoplastic acrylic polymers (TPA) in general have excellent properties including exterior durability. Such resins were widely used in automotive OEM and Refinish topcoats from the 50’s to the 70’s, but their use has dramatically declined due to the high molecular weight necessary to provide properties, they require a high amount of organic solvent to enable air atomized spray application. Accordingly these paints apply at about 20% weight solids. Thermoplastic resins typically use a high level of methyl methacrylate in their polymer backbone to provide excellent hardness and exterior durability.

Figure I – Structure of poly MMA and poly MA

Thermosetting acrylic resins (TSA) are designed with functional monomers to either react with themselves when exposed to heat or moisture, or with that of a cross-linker to form a cross-linked film. Thermoset resins as a group are lower molecular weight and thus have higher application solids. Once cross-linked, as a class they offer films with excellent resistance to organic solvents, moisture and UV light and do not soften appreciably when exposed to moderately high temperatures as thermoplastics do. An example of acrylic monomers with functional groups that can be used to functionalize acrylic polymers to provide properties such as crosslinking, self-crosslinking, improved adhesion or pigment wetting are provided in Table III.

Being able to functionalize an acrylic resin with a wide range of reactive moieties provides the ability to tailor the performance of the resin backbone to provide improved adhesion over a variety of substrates, improved pigment wetting and/or the ability to provide crosslinking or self-crosslinking. Other acrylic monomers are also available to impart sulfonic acid, or phosphoric acid functionality to the acrylic resin.

Carbamate functional acrylics can also be made for example by reacting an isocyanate functional acrylic with hydroxypropyl carbamate. Many of the acrylics in the category of functionalized acrylic resins are used in automotive OEM and refinish clearcoats to provide an excellent combination of mar resistance, chemical resistance and light stability.

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Basics of Alkyd Resin Technology

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Mastering the fundamentals of Alkyd Resin Technology

Although alkyds are no longer the largest volume resin type used in coatings, they still play a significant role in the coatings industry, not only because of their versatility, but also because they employ a significant amount of renewable material.

The term alkyd is derived from alcohol and acid.

Alkyds are prepared from the condensation reaction between polyols, dibasic acids and fatty acids. The fatty acid portion is derived from vegetable matter and thus is a renewable resource. Key performance features of alkyds include their ability to offer improved surface wetting (from the bio-based fatty acid portion of substrates and pigments) and lower cost (also primarily from the fatty acid portion). The most widely used polyols include glycerol, pentaerythritol and trimethyol propane whereas the most widely used dibasic acids are phthalic anhydride and isophthalic acid.

Naturally-occurring oils are in the form of triglcerides. Triglycerides are triesters of glycerol and fatty acids. Triglycerides can be drying oils, but many are not. The reactivity of drying oils with oxygen results in 1,4 –dienes. The naturally-occurring oils are comprised of mixtures of mixed triglycerides with different fatty acids as part of the glyceride molecules.

Some of these glyceride molecules are comprised of a higher percentage of fatty acids with a greater amount of non-conjugated unsaturation with diallylic methylene groups and result in improved drying capability. For example, linoleic acid has one active diallylic group (-CH=CH – CH2 – CH=CH -), whereas linolenic has two active methylene groups. Also, to increase drying speed, alkyds can be modified with vinyl toluene or styrene to increase the Tg and thus reduce the time required to reach a given hardness. If the amount of oil in an alkyd is over 60%, it is called a long oil alkyd. If it’s between 40 and 60%, it’s known as a medium oil alkyd, and those with less than 40 are considered short oil alkyds. The formula for calculating the percent oil length based on the amount of fatty acid is as follows:

Alkyd 2

In addition to the amount of oil as well as the selection of the alcohol and acid functional components, the type of oil has a profound effect on the dry time and performance.

Fatty acids are further categorized into drying, semidrying and non-drying. Non-conjugated oils are considered drying oils if their drying index, as calculated as follows, is more than 70. The higher the amount ofLinolenic and Linoleic content, the higher the drying index: Alkyd 3

Although drying speed is improved as the % linolenic increases, the rate of yellowing for exterior white coatings is also greater. Accordingly, alkyds using safflower and sunflower oils which provide improved resistance to yellowing as a result of their lower linolenic content.

Alkyd 4 Alkyd 5

In addition to classifying alkyds by their oil length and the type of fatty acid present, alkyds are also classified into oxidizing and non-oxidizing categories. Oxidizing alkyds crosslink through a complex multistage auto-oxidation mechanism, whereas Non-oxidizing alkyds do not crosslink and are thus thermoplastic unless their available hydroxyl groups are crosslinked with an aminoplast (heat cured) or isocyanate crosslinker (ambient cured).

To read the rest of the article, written by Chemical Dynamics’ President, Ron Lewarchik, click over to UL Prospector here.

UV-LED Curable Coatings Offer a High-Speed Light Curing Process

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UV-LED Curable Coatings offer a high-speed light curing process with a number of advantages over more conventional cure processes. Multiple advantages include High speed, lower energy requirements, little or no VOC, less production space, less dirt collection, high quality finish, rapid processing as well as instant on-off with some UV light technologies also expedite production and energy savings. UV Curable paint finishes have existed since the 1960’s and are based on polymerization reactions including free radical and cation-initiated chain-growth polymerization. As the majority of coatings for UV cure coating utilize free radical polymerization (>90% of market), this article will focus primarily on free radical polymerization initiated by a photoinitiator (Fig. 1):

The types of unsaturation used in UV/EB cure coatings are provided in Table I, with by far the largest type being acrylate.

Table I – Type of Unsaturation used in UV/EB Cure

Photoinitiator

considerations primarily include two different characteristics of the photoinitiator’s absorption curve. First, is the maximum wavelength (Lambda Max) of light that is absorbed by the PI and second, the strength of this absorption (molar extinction coefficient). Photoinitiators developed for curing pigmented films normally have higher molar extinction coefficients at longer wavelengths between 300 nm to 450 nm than those for curing clear formulations. To maximize cure and efficiency, the PI’s absorbance must match the light output of the lamp as different lamps have different spectral outputs (see Table I). Longer wave- length light is also essential to enhance cure in thicker coatings. Newer PI’s have also enabled the formulation of pigmented coatings in addition to that of clear coatings. The general cure considerations influenced by color, PVC, pigment particle size and film thickness are summarized in Fig. 2:

Figure 2 – UV Cure Considerations. Image: Ciba – Geigy literature

There are two main types of free radical photoinitiators, Type I and Type II. Type I photoinitiators undergo cleavage upon irradiation to form two free radicals. Normally only one of these free radicals is reactive and thus initiates polymerization. 1-hydroxy-cyclohexylphenyl-ketone is a widely used Type I PI. Type II photoinitiators form an excited state upon irradiation, and abstract an atom or electron from a donor molecule (synergist). The donor molecule in turn initiates polymerization. An example of a widely used Type II photoinitiator is benzophenone. Tertiaryamines are typically used as synergists as they react with benzophenone, and also retard the inhibition of polymerization by oxygen. Acrylated tertiary amine compounds are used when odor and extractables are of concern. Oxygen can also inhibit cure especially in thin films; to counteract oxygen inhibition, coatings can use amine synergists, be cured under a nitrogen atmosphere, employ the addition of wax, high initiator concentration, more intense UV Light, and/or surface active initiators.

To read the rest of the article, written by Chemical Dynamics’ President, Ron Lewarchik, click over to UL Prospector here.

Automotive Coating Failure Expert Witness

Expert Witness in Automotive Coating Failure Case

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Automotive Coating Failure Case Expert Witness

Challenge:

A plaintiff in a law suit involving coating failures of a waterborne automotive refinish coating line from a major global paint supplier required an expert witness in coating failures to investigate the claim, provide expert reports, depositions and deliver trial testimony.

Action:

Chemical Dynamics conducted extensive research and testing to assess how the failure occurred and to establish its repeatability. Once failure was proven, Chemical Dynamics provided thorough expert witness support.

Result:

The plaintiff won the case as the jury found the defendant guilty of fraud and misrepresentation of the product’s performance attributes. The plaintiff was awarded a multimillion dollar judgment.

Inert Pigments

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Inert Pigments: The Unseen Contributor to Improving Paint Performance

Inert pigments absorb nearly no light, and therefore, by themselves in a cured paint film, do not stand out from a color perspective. Inert pigments have a refractive index similar to that of the vehicles used in paints, so they provide very little light-scattering. However, used in conjunction with opacifying pigments, they can provide enhanced opacity at lower cost. Inert pigments are also called fillers or extenders as they are normally lower in cost and occupy volume in the paint film. Other valuable functions they provide include improved mechanical properties, rheology adjustment, gloss adjustment, and enhanced barrier protection.

Critical Characteristics of Inert Pigments that Influence Paint Performance

Mineralogy – Chemical composition, crystal structure, Hardness in Mohs (Fig. I)
Physical Characterization – Brightness, refractive index, pH, inertness, oil absorption, purity and presence of soluble salts
Particle Metrics – Particle size, shape, size distribution and aspect ratio

Per Figure I, talc would be a better filler pigment to improve sanding characteristics in a primer-surfacer, whereas a silica based pigment such as quartz (SiO₂) would provide better scrub resistance in an interior architectural wall paint due to increased hardness.

The Chemical composition of a pigment can also play an enormous role in determining the overall impact on the performance. For example, calcium carbonate in exterior latex paint can degrade in the presence of acid rain, producing carbon dioxide and calcium bicarbonate, which is water soluble. This in turn causes the film to be porous and the calcium bicarbonate to migrate to the surface of the paint film, forming a light frosting of insoluble calcium carbonate.

Pigments that have a pH of less than 7 can exacerbate corrosion when used in metal primers. Aluminum in a pigment contributes to the acidity, whereas calcium, potassium, barium, and sodium provide alkalinity. If a pigment contains soluble salts, these salts can contribute to blistering when exposed to moisture.

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Beat the Heat with Solar Reflective Coatings

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Solar Reflective Coatings- a Deep Dive

According to EPA statistics, approximately $40 billion is spent annually in the U.S. to air-condition buildings. The incorporation of solar reflective pigments in paint can decrease the cost to air condition buildings in the U.S. by more than $8 billion.

Figure1: SolarReflective — Figure1: Solar Reflective

When exposed to sunlight, it is commonly known that light colors, especially white, remain cooler than darker surfaces. Darker colors, especially black, absorb infrared light energy, resulting in warming of the substrate. The amount of light energy absorbed is dependent on color.

Other factors that determine an object’s temperature in an outside environment, in addition to it’s color and solar reflectivity, include it’s emissivity, convection and conduction. To illustrate further, Figure 1 indicates at an ambient air temperature of 20° C, a white object will remain at about 20°C, whereas a black object will be about 35° C for the coating surface of a steel building.

However this same black coating will have a surface temperature of about 65°C if the coated substrate is wood, plastic or isolated steel sheets. Key definitions follow:

Total Solar Reflectance – the amount of solar radiation that is reflected by a surface, measured as a percentage.

Thermal Emittance – the ability of a material to dissipate heat away from itself, or rather, to shed heat.

Convection – exchange of energy with air above the substrate.

Conduction – exchange of energy with the layer of the substrate directly below the surface.

To better understand the phenomena of why colors display different heating/cooling characteristics in sunlight, it is essential to examine the natural light spectrum. Solar radiation reaches the earth’s surface in three distinct wavelength packets. These packets of light include Ultra Violet light (UVA 280 – 315 nm, and UVB 315 – 400 nm), Visible light (400 nm – 700 nm) and Infrared light (near IR and far IR) between 700 and 2500 nm. The human eye sees light primarily in the visible portion of the light spectrum resulting in color. Some animal species, such as birds can see light in the UV portion of the spectrum.

Figure 2 illustrates the natural spectrum of solar radiation. 5% of the light energy is UV light, 46% is visible light, and the remaining 49% is infrared light energy. Pigments can absorb or reflect solar infrared energy resulting in 1) heat build-up of the coated substrate if the pigment absorbs IR energy (for example conventional darker pigments); or 2) little or no increase in temperature if the pigment reflects IR light (for example white and lighter colors).

Solar infrared energy (700 – 2,500 nm) is different than infrared energy emitted by hot objects in interior spaces, such as heaters. Infrared energy is found in the far infrared range beyond 1,200 nm.

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How to Protect Against Corrosion

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In 2013, the direct cost of corrosion was 3.1% of the 15.1 trillion in U.S. GDP, which in June 2013 is estimated to equal $500.7 billion. Corrosion is a an electrochemical process where the metal is oxidized by virtue of interaction with its environment, which results in the metal returning to its most stable oxidative state. This article will discuss those factors that influence corrosion, especially in regard to the use of coatings designed to protect the metal to which they’re applied. Accordingly, consideration of the fundamental factors that influence corrosion processes as it relates to the use of organic coatings will be considered herein.

Metals desire to be in their most thermodynamically stable state, which, in simplified terms, is the naturally occurring state of matter in its lowest energy state. Metals ordinarily exist naturally as oxides (e.g. iron oxide, aluminum oxide, zinc oxide etc.), because oxides represent their lowest energy state. Oxidation occurs at the anode (positive electrode) and reduction occurs at the cathode (negative electrode). Corrosion is normally accelerated by the presence of water, oxygen, and salts (particularly of strong acids).

Figure I lists a series of metals and their ability to resist corrosion. The most common metals used in industry include steel (cold rolled and hot rolled steel), aluminum, galvanized steel (hot dip and electrogalvanized steel) as well as galvalume. The latter two metal substrates utilize either a zinc layer or an aluminum/zinc layer respectively on the surface of the steel to enhance corrosion resistance.

Even though aluminum and zinc are less noble than steel, when not coated with an organic coating, they provide longer-term improved corrosion resistance than steel. When steel rusts, the corrosion product (ferric oxide) is loosely attached to the surface, whereas in the case of aluminum or a zinc/aluminum alloy, the corrosion products form a more tightly knit adherent layer to the metal surface that decreases the subsequent rate of corrosion (Table III).

Table III – Corrosion Loss of Uncoated Metals in microns/year in Various Environments. Exterior Durability of Organic Coatings, Eric V. Schmid, FMJ International, 1988 — **Table III – Corrosion Loss of Uncoated Metals in microns/year in Various Environments.** Exterior Durability of Organic Coatings, Eric V. Schmid, FMJ International, 1988

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Waterborne Resin Technology

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Waterborne resin technology is the largest type of coating technology used on a global basis and is expected to continue to grow as a percent of the total coatings market. By 2022, the global market size of waterborne coatings is expected to be over $146 billion USD. Growth in large part is due to increased volume in the construction and automotive markets with acrylics being the largest single type of waterborne resin system representing over 80% of the total waterborne market.

Driving forces for the increased use of waterborne coatings include:

Lower VOC
Ease of cleanup in most cases
Decreased fire hazard
Lower insurance cost
Lower energy use for baked coatings due to the need for less oven make up air
The need for decreased levels of petroleum-based materials.

As the May 2014 Prospector article on Flow, Leveling and Viscosity Control in Water-Based Coatings indicates, the two largest classes of waterborne coatings include water-reducible and latex, with the majority of baked coatings falling in the first category with most of the architectural coatings belonging to the second category. The term water-reducible is used for resins made in solvent and reduced in water to form a dispersion of resin in water. Latex resins on the other hand are prepared by emulsion polymerization in water.

Disadvantages for the use of waterborne coatings include:

High dependence of evaporation rate on relative humidity
High heat of evaporation for water requires 2260 J/g for water and for example only 373 J/g for 2-butoxyethanol, a commonly used cosolvent
Nonlinear viscosity reduction curve for coatings using water reducible resins
High dependence of flow and appearance on relative humidity
High surface tension of water (poorer wetting) requires the addition of surfactants which in many cases detracts from humidity resistance
Waterborne coatings are more corrosive than solvent born coatings and thus require lined containers, plastic or stainless steel to avoid rust
Waterborne coatings are more prone to popping in baked applications as film formation begins to occur before water evaporates from the film (see Table I)

However the continued advancement in material science to include innovations in resin chemistry, surfactants, wetting agents and flow agents will help enable the continued growth of waterborne coatings.

Figure I represents the various stages in drying of a latex based paint system. The first stage involves the evaporation of water. The second stage includes the continued evaporation of water and cosolvent to the point where the latex particles touch and begin to coalescence to form a film that is partially dried.

The final stage involves the continued coalescence and cure (in a crosslinked system) to form a cured, dry adherent paint film.

One of the key considerations in the use of waterborne coatings is the increased role that humidity in addition to temperature plays in the application and cure of these coatings. For example, to provide acceptable application properties, both the temperature and humidity must be carefully controlled as illustrated in Figure II. The effect of humidity on coatings containing water-organic solvent can not be ignored.

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Coil Coating Explained

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Coil Coating Process Analysis

Over 800 million tons of coil-coated metal are produced and shipped annually in North America alone. Coil coating (see Diagram I) is a very efficient way to produce a uniform, high quality, coated finish over metal in a continuous automated fashion. Coil coating is also referred to as pre-painted metal, because the metal is painted prior to, rather than after, fabrication.

In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape. Versus most other application methods, coil coating efficiency is nearly 100%. Application is at very high line speeds as modern coil lines can run at speeds as high as 700 feet per minute and cure the applied paint in 15 – 45 seconds. As opposed to a spray-applied coating, for example, a coil-coated, formed surface offers uniform film thickness rather than the thicker films on edges, corners and bends that is more typical of spray-applied coatings.

Coil Coating Process Explained — Diagram I – Typical Coil Coating Line. Click to view source.

Topcoats

Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.

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Green Resin Technology – The Other Approach

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Green Resin Technology Explained

In April 2015, Chemical Dynamics published an article in Prospector on green resin technology using bio-based resin building blocks in the synthesis of polymers for coatings. Another highly desirable approach to green technology is the incorporation of resins utilizing building blocks derived from recyclable materials.

Sales for resins/polyols using recycled or bio-based polyols are expected to grow twice as fast as the overall polyol market in the next four years[1]. Driving forces for the use of recycled materials in the manufacture of resins can be reduced health hazard[2] (figure 2), as well as environmental and economic factors. Other attributes include reducing the carbon footprint[3] (Figure 1), increasing sustainability, and conservation of natural resources. Green products are also growing in favor with multiple government and private agencies.

Typical recyclable material sources may include polyethylene terephthalate (PET), designated rPET for recycled PET, recovered cooking oils and recycled polyurethane foam. PET is typically used as containers for soft drinks and water, whereas polyurethane foams are used as carpet underlay and in mattresses. In the U.S. alone, there were 6.5 billion pounds of unrecycled PET-based containers in 2013.

EPA Guidelines

The U.S. Environmental Protection Agency Comprehensive Procurement Guideline Program (CPG) defines recycled material as such that the EPA deems equivalent to virgin material. RCRA Section 6002 also requires purchasing agencies to establish procurement programs for designated items that meet CPG. Scientific Certification Services (SCS) recognizes products made either in whole or in part from recycled waste material in place of virgin materials. Through its certification process, SCShelps products qualify for credits within the LEED rating system. LEED is a certified U.S. Green Building Council program. Recycled content is certified by the U.S. Green Building Council’s GreenCircle for total recycled content based on pre and post consumer recycled content in products.

To read the rest of the article by Ron Lewarchik, President of Chemical Dynamics, please click here.