Alkyd is a special term that refers to a particular type of polyester resins which are modified with fatty acids ]1[. They have been used as binders for paints since the 1930s. Alkyds are widely used for producing expansive range of coatings due to their low cost, possibility of synthesizing them from different naturally occurring products, miscibility with variety of polymers, suitable surface properties, good gloss, suitable interaction with polar surfaces like steel or wood, and very diverse formulation options. These include do-it-yourself paints and lacquers for wooden and metallic surfaces, paints for marking roads, anticorrosion paints, architectural and industrial finishes, industrial maintenance, nitrocellulose varnishes, curing coatings-based acids and two-parts isocyanate curable coatings, stove glazes, etc. One of the outstanding characteristics of alkyds is the opportunity of developing a wide variety of products with the same chemistry [2, 3]. Based on oil length, alkyd resins are classified in to long oil, medium oil and short oil alkyds that affects their properties and final applications .
Now a days, the extensive use of Petro-based chemicals and disposal of non-biodegradable waste plastics by burning them have been led to the worldwide climate changes, water pollution and emission of harmful gases. Using natural polymers as a potential alternate for solving these issues cannot be a complete solution because of their limited variety, physico-chemical and mechanical properties. However; bio-based products prepared from natural sources raw materials have attracted great attention due to their renewability, eco-friendliness, availability, degradability, cost effectiveness, toughness and acceptable mechanical properties etc. Growing use of renewable materials can develop a sustainable solution to existing global problems with plentiful environmental benefits. Most of the raw materials which are using for the preparation of the alkyds originate from natural renewable sources excepting for petro-origin phthalic-anhydride. Thus, making bio-based degradable polymers turns alkyd resins into interesting coatings and binders from an environmental viewpoint . One of the desired demands of coating manufacturers is enhancement of the degradability of resins without too much changes in the formulation of their backbone's monomers and properties. This can be almost achievable using suitable modified nano materials. However, another environmental concern of the alkyds is solvents used for adjusting their viscosity. Typical conventional solvent-based alkyds are mixed with toluene, xylene, white spirit or a combination of them. The evaporation of these organic volatiles (VOC) throughout the usage and drying course of alkyds lead to numerous ecological issues . Thus, alkyd resins manufacturers have had to develop more naturally being form of their productions to compete with the customary thermo-plastic lattices that introduced to the market in the 1950s. Recently quite wide activities have been done for designing alkyd emulsions, acrylated alkyd emulsions, water thinned-able alkyd resins, high solid content alkyds and other water-based binders which can be used as binders for eco-friendly purposes [2,3]. For water-based alkyds one procedure is synthesizing high acid number alkyds that after neutralizing by amines and mixing with water and water mixable solvents such as glycol ethers, can be changed to a colloid solution. But the presence of amine results in slow drying and low yellowing resistance. Owing to the colloid character of the water thinned-able alkyd paints, systems exhibit an abnormal viscosity when diluted by water which causes unpleasant problems throughout their usage. In additions, the high pH of the colloidal system (typically pH ≥ 8) results in gradual hydrolyzing of the ester groups which negatively affects polymer molecular weight and their soring durability. These reasons have been resulted in minor success to the commercialization of these systems. Another solution for formulating zero VOC paints can be alkyd emulsions. With the right choice of surfactants, the use of amines is avoidable in alkyd emulsions . The modification of alkyd resins by acrylates combines the favorable application and film formation properties of the alkyds together-with the weathering resistance and overall characters of acrylic systems. However, acrylation of alkyds performs via post-acrylation or monoglyceride method. These reactions add extra synthesis steps to alkyd production and change the formulation of the virgin alkyd. Thus, the preparation of high solids alkyd paints maintains alkyd resin originality as well as solving the high VOC content problem but causes the problem of increasing viscosity by decreasing of diluent solvent contents. There may be many strategies to solve this matter. For example, producing high solid content alkyds by reducing their molar mass through enhancing the OH/COOH groups ratio or enhancement of fatty acids contents. However, alkyd resins prepared via these methods show unsatisfactory properties because of their low molecular weight. Their drying time is long and their coatings performance is weak. One of the suitable ways to solve this issue has been synthesizing highly branched alkyd resins which lead to fast drying coatings with satisfactory properties for decorative end use .
On the other hands, alkyd coatings bear some short comings, like low alkaline and salt solutions stability, extended drying periods (particularly for non-drying oils) and low hardness . Under these situations, paints show extensive chalking, writing, color diminishing or gloss loss. A good strategy for increasing paints’ durability has been improving their weathering resistance, solvent resistance, thermal resistance and etc. via modifications of alkyds with other resins . One way for the improvement of the aforementioned properties is making hybrid alkyds by mixing and blending alkyd resins with other suitable resins like acrylates, polyurethanes etc. because of their suitable miscibility with different kinds of resins [6, 7]. Another way with no change in the alkyd resin chemistry can be using the benefits of nano technology. SiO2, Al2O3, TiO2, ZnO, CaCO3, Fe2O3 and organo clay are nanoparticles which have been widely used for the polymeric coatings [3, 8]. The resulting polymeric coating nano-composites have exhibited greater mechanical, electrical, thermal and chemical properties compare to pure one [3,9,10]. CNTs (Carbon nanotubes) have gained great consideration because of their exceptional mechanical, thermal and electrical properties which result in their widespread usages [8,11–13]. CNTs/polymer nanocomposites show improved thermal and electrical conductivity and strengthened adhesive and cohesive properties of coatings . However, owing to their large surface area together with strong van-der-Waals interactions between CNTs, they can’t disperse uniformly in the polymeric matrix and tend to aggregate together. Thus, in order to provide a well dispersed CNTs/polymeric nanocomposite their surface modification with suitable chemical groups for attending desired properties is helpful [11–15 [. On the other hands these attached groups on the CNTs’ surface can lead to new interactions between modified CNTs and polymer matrix resulting in generating new properties.
In this study we tend to synthetize a new medium oil alkyd resin (Alk) based on soya beans fatty acid using a one-step fatty acid method. Then two kinds of surface modified Carbon nanotubes (CNTs): “stearic acid modified” (CNTs-sa) and “soya beans fatty acid modified” (CNTs-fa) are prepared and incorporated into the alkyd resin. Then different properties of these nanocomposites such as thermal stability, drying time, gel content, hardness, chemical resistance and anticorrosive features, and hydrophobicity are studied and compared. In the present study, high solid alkyd resins containing CNTs modified with a saturated non-drying fatty acid (sa) and an unsaturated drying fatty acid (fa) with a good (not excellent) drying speed are prepared for the first time. Our objective is to prepare developed alkyd resins using the same chemistry and maintaining alkyd originality. It is expected that unlike acrylic, polyurethane, phenolic modified alkyds, amine containing alkyds or high acid value alkyds, without decreasing of alkyd’s molecular weight, changing its structure or chemistry altered alkyds with improved chemical and solvent resistance, good durability, thermal stability, drying time, gel content, hardness, and hydrophobicity are obtained. Although we have used non-drying or medium speed drying oils, these fatty acids result in good- excellent gloss and non-yellowing on aging properties, instead. Thus, we try to improve thermal and coating properties of resins as well as their drying time and gel content using the benefits of CNTs nanotechnology without changing the alkyd resin originality and using low cost, available and biologically renewable raw materials. We tend to prepare ecofriendly low VOC high solid alkyds with relatively unchanged viscosity, improved water-proofness as well as biodegradability.