The term “Matte” refers to the paint of non-glossy finish. The history of matte is
new and can be traced back to the 18th century. Back in 18th century, a certain interest in matted surfaces for furniture arose in the paint and coatings industry. Nevertheless, without some advanced technology or sufficient knowledge about the matte surfaces, obtaining the rough surfaces required much work. Moreover, people in Asia countries tend to choose glossy surface for their exterior or interior decorations since gloss means precious and new. However, in recent years, more and more consumers prefer matte to gloss for their interior decorations such as the furniture. Their choice is based on the issues regarding clean-ability, haptic qualities and personal aesthetics. The skyscraper is a classic example to illustrate the importance of matte surface. The panel of the skyscrapers are coated with gloss matting paint in advance to avoid the hazardous reflections into the eyes of pedestrians. Some applied coatings are matte for the sake of economic issues. The scratches on the coating can hardly be seen owing to the antireflection of the matte surfaces. Consequently, the market growth of the matting agent product is expected to grow rapidly in the following years owing to the rising consumer demand.
Owing to the fast growing market in matting agent and the furniture is essential for every household, this study aims at developing a novel gloss matting paint for the woodenware.
Chapter 2 Literature Reivew
2-1 Gloss introduction and gloss measurement
2-1-1 Gloss Introduction
Gloss is the power for a surface to reflect light specularly. If light incident on the surface is reflected specularly more, the impression of gloss will be more obvious. There are two terms usually mentioned in the field of gloss measurement including specular reflection and diffuse reflection. Specular reflection is reflection which causes surfaces to exhibit high lights and to appear somewhat like a mirror. On the contrary, Diffuse reflection is reflection which causes a surface to possess lightness or darkness of some degree which looks gray.1
The effects of gloss are determined by the interaction of light with the surface and the physiological evaluation scale. As shown in Figure 1-1, regarding the interaction of light with the surface, the incident light is mostly reflected in the main direction of reflection for the smooth surface. Specular reflection occurs more frequently than diffuse reflection. On the rough surface, the light is scattered in all directions leading to diminished image forming qualities. In other words, If more light is scattered, the reflection in the main direction is less intense. Diffuse reflection occurs more frequently
than specular reflection.
2-1-2 Gloss measurement
On the aspect of physiology, human eyes play the critical role in the evaluation of gloss difference. However, the perception of the appearance of the surface is subjective and differs from person to person. In consequence, defining the appearance of the surface with objective and measureable criteria is essential that it takes an instrument to measure the gloss of the surface. Moreover, the human eyes can only tell the difference of 3 GU(gloss unit) on the matte surface. The human eyes can’t tell the difference of 3GU on the glossy surface. On the contrary, the instrument that measures the gloss can tell the difference of 0.1GU on both the matte and the glossy surface. This difference can also help assure the quality of the product surface. If the gloss of one part of the surface is quite different in figure but looks similar to other parts of the surface, the defects on the part of the surface can be found by the gloss instrument.
Light source
Figure 1-1 Surface structure and gloss.
As Figure 1-2 shows, the gloss instrument (glossmeter) can be categorized into three kinds (20o, 60o, 85o) by the degree of the sensor and light source to the surface.
Figure 1-2 Different kinds of glossmeters (20o,60o,85o).
The instrument is composed of the light source and the light sensor. Take 60 o degrees to illustrate how the glossmeter measure the gloss of the surface. As shown in Figure 1-3, the light by the glossmeter is incident on the surface by 60 o degree to the surface. The sensor is placed in the position which is 60 o degree to the surface to receive the reflected light from the surface. The measurement results of a glossmeter are related to the amount of reflected light not the amount of incident light.
Figure 1-3 Illustration of gloss measurement by the glossmeter.
60o 60o
As Table 1-1 shows, the 85 o degree glossmeter is more sensitive to differences in gloss below 10 GU while the 20 o degrees has higher resolution on high gloss coatings (above 70 GU). The 60o degrees is suitable for measuring the coatings with gloss 10-70 GU. 60 o degrees is chosen for our gloss measurement of the substrates coated with gloss matting paint since gloss criterion for the commercial gloss matting paint is 10-20 GU.2
Table 1-1 Different gloss ranges and corresponding measurement degrees.
Gloss Range with 60° Gloss Meter Measure With
Semi Gloss - 10 to 70 GU 60 ° High Gloss > 70 GU 20 ° Low Gloss < 10 GU 85 °
2-2 Introduction to mechanism of matting
It is widely known that the surface roughness is the cause of low gloss or matte surface for the surface.3 The phenomenon of matting is commonly thought of as reduction in the amount of specular reflection arising from the surface roughness. This phenomenon can be illustrated and explained by the film shrinkage and drying process. The matte paint film formation process is shown in the following schematic figure for better understanding of this phenomenon.4
As shown in figure 1-4, the matting agents (gloss matting latex particles) are initially uniformly distributed in the film comprised of solvent, matting agents and other additives.
As the solvent evaporates, the film shrinks step by step, which leads to the dropping of the matting agent to the bottom of the film. The dropping of the matting agents are hindered by the viscous or elastic forces due to the solvent evaporation. However, the dropping forces exerted on the matting agents are still large enough to maintain the dropping motion.
A level surface is maintained due to the dropping of the particles and the surface tension force exerted on the surface. After a certain amount of time of film shrinkage and drying, the levelling effect of surface tension is no more able to maintain a level surface
against the viscoelastic forces. As a result, Movement of the matting agents are hindered, continued loss of volume results in the particles staying around the surface and a rough surface is the result.
Fumed silica is widely used in the development of commercial gloss matting paint for its low price and availability.5 Nevertheless, the disadvantage of fumed silica is the poor resistance to the scratch for the film containing silicas due to the incompatibility of silica and the glossy paint latex particles. Since a polymer is more compatible with another polymer, I tend to synthesize a novel polymer particle that possesses gloss matting ability to replace the commercial fumed silica.
Dry film Liquid coating
Figure 1-4 Film shrinkage and drying process.
Solvent Evaporation
Micro-roughness created as film shrinks
Matting agent
2-3 Introduction of DPnB
DPnB is a kind of coalescent or a kind of plasticizer that is blended with the glossy paint latex or gloss matting latex for better film formation performance. A plasticizer can pose a change to the thermal and mechanical properties of the original polymer which it is added into. These changes include increase of the toughness, increase of the elongation to break at room temperature, lowering of rigidity at room temperature, increase of polymer chain flexibility.6 The general requirements for the plasticizer are low volatility, temperature stability, no odor, high degree of solvent power for the polymer, high compatibility with the polymer and so on.
According to the technical data of the commercial product DPnB, it is a slow-evaporating, hydrophobic glycol ether which possesses excellent surface tension-lowering ability and coalescing properties. DPnB is a relatively slow-evaporating solvent in water-borne latex systems. DPnB boosts for the following four properties: (1) high polymer plasticizing efficiency (2) great polymer mobility contribution (3) relatively slow evaporation rate. Since DPnB is compatible with the water-borne latex and possesses the above properties, it’s a good option for lowering the minimum film formation temperature (MFFT). The DPnB (3.5wt% of the glossy paint) is blended with the glossy paint latex and the gloss matting latex for better film formation.
2-4 Introduction of core-shell type particles
Core-shell nanoparticles have attracted great attention among the academic field in the recent years owing to its diverse applications in many fields. Core/shell nanoparticles are multifunctional materials by combing the properties of both the core and shell.
Changing either the comprising materials or the core to shell ratio can modify the properties of the core shell nanoparticles.7 One of the advantages of core-shell nanoparticles compared with directly blending method of fillers is the increase in the interfacial adhesion between the core and the shell. This assure the location of the embedded fillers leading to less aggregation during the blending process.8
The aims of coating shell materials on the core particle are many fold, inclusive of increasing the functionality and stability, reduction in consumption of expensive materials and surface modification.9 Core shell nanoparticles can be categorized into four groups by the material properties of the shell and the core: (i)inorganic/inorganic (ii) inorganic/organic (iii) organic/inorganic (iv)Organic/organic.
The third type of core shell nanoparticles have dual properties of both the inorganic and organic materials.10 These particles exhibit polymeric properties such as flexibility and toughness due to the core material properties. On the other hand, thermal stability
and resistance to oxidation are also properties of these nanoparticles owing to their inorganic shell.11 The core particles can be polystyrene, polyurethane, poly(ethylene
oxide) or some copolymers while the shell can be made from metals or silica. The organic/inorganic core shell nanoparticles have found its application in paints, catalytic, electronic and biomedical fields.12
2-5 Introduction of soapless emulsion polymerization
In the process of emulsion polymerization, the use of surfactants is an well-known procedure.13 However, for many purposes, there are some disadvantages of this technique, including, the pollution to the environment, the calibration of instruments, and the determination of particle sizes. The removal of the surfactants from the systems will lead to coagulation. In addition, the surface charge in the electrophoretic measurements will be affected by the adsorbed surface layer .Consequently, the polymerization of latex particles in the absence of emulsifier (surfactants) is an efficient way to solve the problems encountered above. The dispersion stability of the latex particles synthesized by the emulsifier free polymerization is mainly attributed to the cationic or anionic functional group originated from the initiator.14
In the soapless emulsion polymerization, the monomer forms large monomer droplets in the mixture and the droplets supply proper amounts of monomer to the sites where the polymerizations takes.13 There are two types of polymerization mechanisms for soapless emulsion depending on the solubility of the monomer.
For the slightly soluble monomer, like methyl methacrylate, some monomer and at least one ionic group which derives from the initiator (sulfate group for KPS initiator)
form oligomer radicals .As it combines with more monomer, the radicals will precipitate because of its decreasing solubility and later becomes the loci of polymerization. The oligomer radicals acts as the protection for the monomer and other hydrophobic species resulting from the hydrophilic ionic end groups. Shorter-chain oligomers or monomer will be more likely to be incorporated into the inner part of the oligomer radicals and a latex particle will be formed. These latex particles will aggregate into groups owing to their colloidal instability. The aggregation process will be repeated until the repulsion due to the surface charge on the particles can stabilize the particles. The latex particle will continue to grow as long as free radicals and monomer are available. This is so called homogeneous nucleation. On the other hand, for highly-insoluble monomers, like styrene, the oligomer radicals terminate first with the surface-active oligomers and precipitate.
The nuclei and surface-active oligomers collide with one another to form larger latex particles. Similar to the case mentioned above, the particles can be stabilized by the repulsion of the surface charge of the particles. This is so-called the micelle nucleation.
There are some characteristics for the soapless emulsion polymerization, including the monodispersity of particle size, smaller molecular weight compared with emulsion polymerization and longer time for polymerization to complete. Since the latex particles which form during the soapless emulsion polymerization form at the beginning of the
polymerization, the particles will be uniform in size.15 Due to the absence of emulsifier, the soapfree emulsion polymerization usually takes longer time than the emulsion polymerization to reach high conversion rate.
2-6 Introduction of sol-gel method
Sol-gel method has been widely used both in the industry and in the academic field.
The start of the research on this method can be traced back to 1820s when Ebelman, and Graham's yielded Si02 in the form of a "glass-like material " using TEOS as the reactants under acidic conditions.16
Sol is a suspension of colloidal particles that exhibit Brownian motion in a liquid.
Colloid refers to small particles with diameters of 1-1000nm. Gel is an interconnected network with porous structure whose dimensions is about submicron and the polymer chain with average length greater than a micrometer.17 The main reactions that will occur in the process of sol-gel method includes hydrolysis and condensation which will be further discussed in the following passage.
The advantages of sol-gel are as follow: (1) Highly pure products can be obtained by removing the precursors via distillation or other simple purifying methods (2) The low reaction temperature can prevent the materials from reacting with the container walls (3) The shape and size of the colloidal particles can be precisely controlled by varying the experimental conditions.(4) Being Capable of nano-sized catalysts with large specific areas.18
When it comes to sol-gel method, hydrolysis and condensation are always the first to be mentioned. Silicon alkoxide is widely used in many aspects. Therefore, it is taken to illustrate the hydrolysis and condensation reaction of sol-gel method.
A. Hydrolysis
An alkyl group on the silicon alkoxide is hydrolyzed by the water in the reaction medium to become an alkoxy group. This reaction is called hydrolysis. The number of alkyl groups hydrolyzed is determined by the amount of catalysts and water present in the reaction medium.
B. Condensation
A water or alcohol molecule is removed from the silicon alkoxides with at least one alkyl group hydrolyzed to form the Si ≡ O ≡ Si bonding. This reaction is called
condensation. The reaction rate of condensation step is determined by the functionality of the monomers and the amount of water present in the system.
There are many factors affecting the hydrolysis and condensation rate, including the pH value in the system, the steric and inductive effects, the solvent effects. Each of them will be introduced in the following paragraphs.
(1) The influence of the pH value
In the process of sol-gel reaction, the pH value of the system can make some differences in the optical properties, reaction rate and the size distribution of the colloidal particles. Under acidic conditions, the silicon alkoxides are more likely to be hydrolyzed owing to the faster hydrolysis rate. However, the condensation rate for acidic conditions is much slower. The fast hydrolysis rate and slow condensation rate lead to particles of smaller sizes. On the contrary, the condensation rate is faster than the hydrolysis rate leading to particles of larger size. This phenomenon is shown in the figure 1-5
Figure 1-5 The sizes of silica via sol gel method under acid or base conditions.19
The pH value of the system can be divided into three ranges, including pH≤2, pH=2-7 pH ≥pH=2-7, to discuss the influence of different pH values.19
（Ⅰ） pH≦2
When the pH value of the system equals 2 which is the isoelectric point of silica, the rate of hydrolysis is much faster than the condensation rate leading to long gelation time.
When the pH<2, there are a large number of hydrogen ions in the system. The hydrolysis rate is quite fast and the condensation rate is directly proportional to [H+] in the system. The gelation time increases with the decrease of the pH value.
（Ⅱ）pH 2~7 ,
When the pH in the system is 2~7, the slow hydrolysis rate and fast condensation rate will lead to faster gelation rate with the increase of the pH value. The polymerization
rate is directly proportional to [OH-] in the system and the particle size can be up to 2nm.
In addition, the particles formed are inclined to aggregate by collision.
（Ⅲ）pH≧7
When the pH>7, there are more hydroxide ions than hydrogen ions in the system.
Hence, the condensation rate is directly proportional to the [OH-] in the system.
The larger particle size is due to the addition of the monomer and the high solubility for SiO2 under the basic conditions rather than the gelation of the particles by collision.
(2) Steric Effects
The silicon alkoxides with more branches or longer alkyl chains are more difficult to be hydrolyzed due to the steric effects.20 For instance, the hydrolysis rate of the tetramethyl orthosilicate(TMOS) is faster than that of the tetraethyl orthosilicate(TEOS).The hydrolysis rate constants of different tetralkoxysilanes under acidic conditions at 20oC is shown in figure 1-6.20
Figure 1-6 Rate constants of different tetraalkoxysilanes for acid hydrolysis.
(3) Inductive Effects
The mechanism of the silica alkoxides hydrolysis is highly relevant to the inductive effects. The more stable the transition state, the faster the hydrolysis rate. 20 During the reaction of the hydrolysis, the alkoxy group is protonated to a hydroxyl group which is a better electron withdrawing group. Therefore, it will lead to the decrease of the electron
density on the Si atom. Under acidic conditions, the hydrolysis rate decreases with each hydrolysis step, whereas under basic conditions, each subsequent step occurs more quickly as hydrolysis and condensation proceed.
(4) Solvent Effects20
Solvents play an important role in the process of sol-gel method due to the low solubility of silicon alkoxides in the water. Therefore, choosing a suitable cosolvent for dissolving both the water and the silicon alkoxides. In most cases, alcohol is used as the solvent for its low boiling point and high volatility. Ethylene alcohol is the most common one among the alcohol for its availability and low price. Figure 1-7 shows the miscibility between ethylene alcohol, water and the TEOS.21
The amount of solvent in the sol-gel process have a great impact on the sol-gel reaction. Insufficient amount of solvent can lead to the incomplete miscibility between the water and the silicon alkoxides, which will lead to incomplete reaction between them.
Excessive amount of solvent may lead to low concentration of the reactant, which leads
Excessive amount of solvent may lead to low concentration of the reactant, which leads