Low VOC Coatings Research Articles

Acute toxicity of petroleum asphalt seal coat leachates to Ceriodaphnia dubia is linked to polymer preservatives

Low-VOC waterborne asphalt-emulsion (AE) seal coat is considered more sustainable than solvent-based coal-tar emulsion seal coat because asphalt emulsions contain negligible amounts of carcinogenic PAHs and release fewer harmful volatile organic compounds. Yet, many low-VOC coatings leach water-soluble substances under outdoor conditions. To investigate the chemical composition of seal coat leachates, three AE formulations were cured under natural weathering conditions and exposed to simulated runoff over a 10-day field trial. Runoff was collected and concentrated using ion-exchange solid-phase extraction (SPE) and analyzed using gas chromatography/mass spectrometry (GC–MS). Leached compounds included hydrocarbons, esters, amines, siloxanes, plasticizers, biocides, polyethylene glycol (PEG) ethers, urethanes, and toluene diisocyanate (TDI). Glycol ethers comprised 29–97 % of the measured leachate mass. Two seal coat formulations contained isothiazolinone biocides, methylchloro- and methylisothiazolinone (CMIT/MIT; 0.5 mg/L in runoff), while a third seal coat formulation continuously leached TDI, a reactive polyurethane (PU) precursor (0.7 mg/L in runoff). Biocide-containing leachates showed acute toxicity to the freshwater water flea, Ceriodaphnia dubia after 48 h, while TDI-containing leachate showed no acute toxicity, suggesting that leachate toxicity was due to in-can polymer preservatives. As biocides are implicated in impaired reproductive signaling, these results support the use of alkaline pH to avoid biofouling and reinforce the goal of reducing and/or avoiding the use of biocides altogether, especially for environmentally friendly products.

Demand for Low-VOC Coatings Continues to Rise

Demand for Low-VOC Coatings Continues to Rise

Study of Thermal Properties on UV-Curable Coatings Derived from Oleochemical Polyols

Over the last few years, UV radiation curing has become a well established coating technology used for several different applications. Polymeric component based on renewable resource is another area for getting low VOC coatings. In the present study cellulose and soya fatty acid were used to prepare oleochemical polyols which in turn reacted with diisocyanates to get PU which is further reacted with hydroxyl terminated acrylates to get oligomeric urethane acrylates. These polymeric compounds are used without any organic solvent for UV-coating formulation and cured by UV light on the metal surfaces. Thus effect of different types of polyols were used to prepared UV coatings studied on thermal properties like dynamic mechanical properties (DMA) and thermogravimetric analysis (TGA)

Polyols based on isocyanates and melamines and their applications in 1K and 2K coatings

Polyurethane polyols (PUPOs) and melamine polyols (MEPOs), invented by AkzoNobel Coatings, offer valuable alternatives to the commonly used polyester and acrylic polyols and new formulating tools for the development of novel coatings. The resin chemistry is based on the predominantly selective reactivity of α,β- and α,γ-diols with commonly used polyisocyanate and melamine formaldehyde crosslinkers. The resulting low molecular weight, hydroxyl functional resins are suitable for use in low VOC coatings. By choosing the appropriate crosslinker, diol, and modifier, the chemical structures of these resins can be altered to obtain the desired properties of the coatings. Synthesis methods for novel PUPOs and MEPOs and properties of one component and two component coatings containing them are described.

Polyols based on isocyanates and melamines and their applications in 1K and 2K coatings

Polyurethane polyols (PUPOs) and melamine polyols (MEPOs), invented by AkzoNobel Coatings, offer valuable alternatives to the commonly used polyester and acrylic polyols and new formulating tools for the development of novel coatings. The resin chemistry is based on the predominantly selective reactivity of α,β- and α,γ-diols with commonly used polyisocyanate and melamine formaldehyde crosslinkers. The resulting low molecular weight, hydroxyl functional resins are suitable for use in low VOC coatings. By choosing the appropriate crosslinker, diol, and modifier, the chemical structures of these resins can be altered to obtain the desired properties of the coatings. Synthesis methods for novel PUPOs and MEPOs and properties of one component and two component coatings containing them are described.

Dimethyl secondary amine chain extenders: a conceptual approach to in situ generation of advanced epoxy resins for rapid cure, low VOC coatings

Solid epoxy resin oligomers have traditionally been utilized for coatings that combine fast dry-to-touch speed and good flexibility, properties greatly in demand in many applications such as marine and industrial maintenance coatings. Unfortunately, solid epoxy resins require formulation with large quantities of solvent that make the attainment of modern VOC regulations difficult or impossible. Coatings formulated from low molecular weight liquid epoxy resin (LER) on the other hand, can more easily meet VOC challenges, but dry slowly and tend to be brittle. This article explores the concept of using fast reacting, difunctional amine chain extenders to generate epoxy oligomers in situ as a means of meeting these opposing property demands. Methylamine-terminated poly(N-methylazetidine) (p-NMAz) is prepared in a two-step process involving the Michael addition of methylamine to acrylonitrile followed by hydrogenation in a methylamine-containing atmosphere to yield an oligomer stream with an Mn of about 250. Hydrogenation of isophthalonitrile in a methylamine-containing atmosphere yields N,N′-dimethyl-meta-xylylenediamine (DMMXDA). Competitive pseudo-first-order kinetic measurements conducted in isopropyl alcohol indicate these amines react approximately 3–3.6 times faster with phenyl glycidyl ether (PGE) than the primary amine meta-xylylenediamine (MXDA). These chain extenders can be formulated with traditional multifunctional amine crosslinkers to yield coatings with lower VOC, faster dry speed, and better flexibility compared with corresponding coatings formulated without the chain extender. Consistent with their chemical structures, p-NMAz proved capable of yielding coatings with the best impact resistance and mandrel bend properties, while DMMXDA yielded coatings with better water and corrosion resistance properties. Open image in new window

Block copolymers as low-VOC coatings for wood: characterization and tannin bleed resistance

Replacement of solvent-borne primers with low-VOC water-based coatings has created the problem of tannin bleed for cedar siding finishers. Three amphiphilic block copolymers, poly(styrene- b-acrylic acid) (PS-PAA), poly(styrene- b-ethylene oxide) (PS-PEO), and poly(styrene- b-methacrylic acid) (PS-PMAA), were investigated as potential water-based coating components that could create a hydrophobic barrier to block tannin extraction. Resistance to extractive bleed was determined by measuring topcoat color change under ambient and humid exposure of coated wood panels. Contact angles of water on the copolymer-coated surfaces were measured to reveal the relative hydrophobicity of the coating surface. Adhesion characteristics of the experimental coatings on wood substrates were determined with a standard tape test. The block copolymer coatings were found to have less darkening and yellowing of the topcoat than the unprimed control and commercial oil-based primer. They also exhibited bleed resistance similar to that of a commercial water-soluble tannin-blocking polymer, but did not perform as well as a commercial water-based tannin-blocking coating. Most copolymers had contact angles close to or greater than 90°, indicating hydrophobic surfaces, with four coatings exhibiting surfaces as hydrophobic as a commercial oil-based primer. Adhesion of the copolymer coatings was found to be adequate under ambient conditions but poor for substrates exposed to humidity. Thus, the amphiphilic block copolymers appear to be potential waterborne tannin-blocking agents with other properties suitable for wood coatings.

High performance accelerated all acrylic coatings: Kinetics and mechanistic aspects of non-isocyanate coatings

Non-isocyanate coatings continue to be among the most promising technologies used to meet environmental and safety concerns, as well as the performance requirements of the industrial coatings market. In general, the market demands the use of non-isocyanate, low VOC coatings without sacrificing the excellent durability associated with 2K urethane coatings. A novel approach to high performance, low VOC, non-isocyanate coatings has been developed. This approach is based on accelerated all acrylic (AAA) binders containing epoxy, tertiary-amine, and an in situ formation of acid (via anhydride and hydroxyl) groups. A model study to demonstrate the mechanism and kinetics of cure is presented. Three major cure reactions can lead to a highly cross-linked polymeric network. They include: (1) the reaction of anhydride and hydroxyl to generate the carboxyl group, (2) the quaternization of the tertiary-amine, and (3) the formation of oligomers. Coatings film properties have been correlated with the model study. Glycidyl-based epoxies are more reactive under basic conditions, while cycloaliphatic epoxies react more effectively under acidic conditions.

Investigation into mechanism of protection of pigmented alkyd coatings using electrochemical and other methods

This paper reports some work conducted on several different alkyd coatings. Electrochemical techniques particularly the electrochemical noise method (ENM) have been used to monitor the performance of two and three coat systems in sodium chloride solutions. For one of the coatings systems comparison has been made between the results from room temperature continuous immersion exposure and accelerated tests both a cabinet test and a high temperature immersion test and good correlation has been obtained. Detached films have also been examined to obtain mechanism information and to clarify how these alkyd coatings work to prevent corrosion. Both the DC resistance characteristics, the noise resistance and the inhibitive properties of extracts have been examined. These tests appear to give information which is useful in deciding how effective the coating will be both when intact and when it needs to protect at a break or scribe. The work provides pointers for investigations of low VOC coatings from renewable resources such as water soluble alkyd–linseed oil coatings.

Polyorthoesters for low-VOC coatings

A series of new oligomeric polyorthoesters for low-VOC coatings was prepared by protecting OH groups with orthoester compounds to achieve both low viscosity and excellent compatibility with crosslinking agents. These new oligomeric polyorthoesters were synthesized through a co-condensation reaction among polyol, orthoester, and α- or β-glycol compounds under an acidic condition. Here, the α-or β-glycol compounds were used as stoppers to prevent excessive polymerization. The polyorthoesters can easily be hydrolyzed with atmospheric moisture, and the produced OH groups can react with appropriate crosslinking agents. It is unique in this system that even the stoppers, α- or β-glycol compounds, can react with the crosslinking agents to achieve an extremely low VOC emission. The polyorthoesters cured with polyisocyanate compounds proved to be highly crosslinked polymers showing excellent mechanical properties, chemical resistance, etc.

Quantitative assessment of the penetration of water-borne and solvent-borne wood coatings in Scots pine sapwood

In recent years the environmental legislation has been working towards a common goal, namely to reduce VOC emission in all branches of industry. The paint industry also has to make its contributions and is focussing on more environmentally friendly coatings likes of water-borne systems and high solid paints. Profound, comparative research has already been performed in this field, still- the physical characteristics of the new coatings remain a debatable topic. In order to assess the penetration of several water-borne primers into wood, a combination of qualitative and semi-quantitative methods is applied. The used techniques are based on fluorescence microscopy and compare the penetration of several low VOC coatings with the penetrating ability of traditional solvent-borne systems.

Wet adhesion of low-VOC coatings on wood: A quantitative analysis

Adhesion of coatings on wood is most critical under wet conditions. However, the mechanisms controlling the adhesion are not well understood. Therefore, a quantitative analysis of coating adhesion on wood is needed. A new technique is presented to measure the wet adhesion quantitatively based on measuring the forces needed to peel the coating from the wood with a tape. Differences in peel forces corresponded to areas in the wood with a lower or a higher degree of coating penetration. Coatings with better penetration and substrate wetting showed an improved adhesion and the better adhering coatings had an increased level of cohesive failure in the coating. Chemical analysis of the air-faced and wood-faced side of the coatings showed differences in chemical composition. The stored strain energy due to hygroscopic stress could reduce the actual level of adhesion significantly. Surface energy effects did not contribute to reduced adhesion after exposure to water. The interfacial work of adhesion between coating and wood was very much lower than the measured force and lacked correlation with measured differences in adhesion. Therefore, it can be concluded that mechanical anchoring plays a dominant role in obtaining good adhesion of a coating on wood.

Improvements in the application properties of water-based low VOC coatings

This paper discusses a new class of acrylic latex (NeoCryl TX200) in which the colloid stabiliser is designed to interact with titanium chelates such as Tilcom AT23, thereby building up a weak gel structure which can be broken down by shear. This class of latex can be formulated with associative thickener (and their blends) in conjunction with titanium chelates, thus providing considerable formulation flexibility which cannot be achieved with conventional surfactant stabilised acrylic latex. A key feature of this new class of latex is that the colloid stabilisation system does not introduce water sensitivity, and indeed wet adhesion characteristics are fully comparable to those of state of the art adhesion promoted latexes. A further benefit of this class of latex is that of increased open-time, arising in part from its intrinsic rheological characteristics, and in part from the high film build achievable with thixotropic formulations. Rheological measurements show that introduction of titanium chelates greatly increases the low shear-rate viscosity of the system, and vane rotor measurements show that there is a near linear dependence of gel strength with titanate concentration. This gel, which is responsible for preventing in-can settlement of pigments and good brush pick-up, is partially broken down by the action of brush dripping, and is completely broken down during application (eg by brush or spray). The structure subsequently builds up with time, with gelation occurring in as little as 5–10 minutes, but with full recovery of gel strength taking in the region of 10–12 hours. The titanate chelating agent, even at high concentrations, does not contribute to the high shear-rate viscosity of the system, this being controlled by the addition of associative tickeners, either alone or in combination. The associative thickener(s) also control the low shear-rate viscosity during the period in which the gel structure has been sheared out, and thus good flow and levelling properties can be obtained through the optimisation of associative thickener type(s) and concentration(s). Although, as might be expected, cosolvents affect the rheological properties of formulations containing both associative thickener and titanium chelate, this effect is not a large one, and can be readily taken into account during formulation design.

Oligomers in the evolution of automotive clearcoats: mechanical performance testing as a function of exposure

Automotive coatings must provide excellent resistance to chemical and mechanical damage in order to maintain a vehicle's long-term appearance and the owner's long-term satisfaction. The Automotive Industry and coating suppliers are partners in design and delivery of future coatings capable of meeting customer demanded performance. As a result of this partnership, new coating materials are being explored based on oligomer chemistry that show promise in providing improvements in both physical and chemical properties/performance and the long-term maintenance of those properties. Oligomeric systems are also useful in design of low VOC coatings. These supersolids coatings will be capable of meeting current and future air quality standards. In this paper measurement techniques for monitoring chemical and mechanical property changes, including cure rate, crosslinking, tensile properties, rheology and scratch and mar performance, were explored. Laboratory mar tests, wet and dry rub tests, which have been validated by commercial experience, are currently used as the basis for comparison of a coating's mechanical performance. QUV accelerated weathering was combined with micro-scratch experiments, atomic force microscopy, optical microscopy, image analysis and IR surface characterization techniques to provide correlation's between chemical composition and mechanical performance, and an indication of service life.