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Printing material science
POLYMER
Monomer and polymer: A low-molecular-weight compound that can be reacted and united with other low-molecular-weight compounds to form a long, higher-molecular-weight chain-like polymer. The formation of polymers—a chemical process called polymerization—is the basis of the drying of many types of printing inks, commonly preceded by the chemical process of oxidation, and many printing ink resins are produced by the process of polymerization. A monomer is considered the basic structural unit of a polymer, and various smaller combinations of monomers are described as dimers, trimmers, or oligomers.
Homo-polymer and Copolymer: Homo-polymers consist of single species of repeating units whereas copolymers consist of two or more types of repeating units. Homo-polymers have a single type of monomer whereas Copolymers have two or more types of monomers. Homo-polymers usually have a simple structure whereas copolymers have a complex structure.
Copolymer: A large molecule created by the polymerization of two or more smaller molecules (called monomers)
Homo-polymer: A homo-polymer is a polymer made from many copies of a single repeating unit.
Types of Polymer: The polymers are divided into 3 types depending on their source of availability. They are natural, synthetic, and semisynthetic polymers. Natural polymers are present in plants and animals and exist naturally. Proteins, starch, cellulose, and rubber are a few examples. Biodegradable polymers are also called biopolymers. Semi-synthetic polymers are produced from naturally existing polymers and then chemically modified, for example cellulose nitrate and cellulose acetate. Synthetic polymers are man-made polymers. The most prevalent and commonly used synthetic polymer is plastic. It is utilized in a variety of industries and dairy products, for example nylon-6, polyethers.
Natural polymers: They are of two type’s organic and inorganic polymers. Organic polymers are important in living organisms because they provide basic structural components and participate in key life processes. Polymers, for example, make up the solid components of all plants. Among them are cellulose, lignin, and different resins. Cellulose is a polysaccharide, which is a polymer made up of sugar molecules. Lignin is made up of a complex three-dimensional network of polymers; wood resins are polymers of isoprene, a simple hydrocarbon. Rubber is another well-known isoprene polymer. Proteins, which are polymers of amino acids, and nucleic acids, which are polymers of nucleotides—complex molecules consisting of nitrogen containing bases, sugars, and phosphoric acid—are two other major natural polymers. In the cell, nucleic acids convey genetic information. Starches are natural polymers made of glucose that are essential sources of nutritional energy supplied by plants. Many inorganic polymers, such as diamond and graphite, can also be found in nature. Both are made of carbon. Carbon atoms in diamonds are bonded in a three-dimensional network, which gives the substance its strength. Carbon atoms join in planes that may glide across one another in graphite, which is employed as a lubricant and in pencil leads.
Synthetic polymers: Synthetic polymers are produced by a variety of processes. Many simple hydrocarbons, such as ethylene and propylene, may be converted into polymers by adding monomers to the developing chain one after the other. Polyethylene is an additive polymer comprised of repeated ethylene monomers. It might include up to 10,000 monomers linked together in long coiled strands. Polyethylene is crystalline, transparent, and thermoplastic, which means that when heated, it softens. It is used to make coatings, packaging, molded components, and bottles and containers. Polypropylene, like polyethylene, is crystalline and thermoplastic, but it is tougher. Its molecules can be made up of 50,000 to 200,000 monomers. This compound is utilized in the textile sector as well as in the manufacture of molded products.
Classification of polymer based on structure:
Polymerization
2.3.1 Linear, Branched, and Cross-linked Polymers
Polymeric materials could be linear, branched, or cross-linked subjected to the intermolecular linkages between the individual chains. The chain structures of linear, branched, and cross-linked polymer.
Schematic representation of linear, branched, and cross-linked polymers.
2.3.1.1 Linear Polymers
In linear polymers the repeating units are joined together end to end in a single flexible chain. The polymeric chains are kept together through physical attractions. These polymers have extensive Vander Waals attractions keeping the chains together. Typically linear polymers are made from monomers with single end group. Linear polymers containing side groups as part of monomer structure do not qualify as branched polymers. Some of the common examples of linear polymers are polyethylene, PVC, polystyrene, and polyamides. Linear polymers are generally more rigid.
2.3.1.2 Branched Polymers
Branched polymers have side chains or branches growing out from the main chain. The side chains or branches are made of the same repeating units as the main polymer chains. The branches result from side reactions during polymerization. Monomers with two or more end groups are likely to support branching. For a polymer to classify as branched polymer the side chains or branches should comprise of a minimum of one complete monomer unit. One of the most common example is low-density polyethylene (LDPE) and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials.
Branched polymers display lower density as consequence of reduced packing efficiency of the branched chains. The length of the side chains or branches differentiates between long- or short-branched polymers. Long branches could have comb-like, random, or star-shaped structures. While the branches may in turn be branched however, they do not connect to another polymer chain.
2.3.1.3 Cross-linked Polymers
Cross-linked polymers, as the name suggest, are polymers in which the adjacent polymer chains are connected in a three-dimensional network structure. The connections are also known as crosslinks. The crosslinks could be a consequence of covalent bonding between the chains or branches. The structure produced. Crosslinks tend to be permanent in nature. Once the crosslinks between the chains develop the polymer then becomes thermoset. Such polymers are characterized by their crosslink density or degree of crosslink which is the indication of number of junction points per unit volume. Common examples include epoxies, bulk molding compounds, rubber, and various adhesives.
Polymerization:
Polymerization is the process to create polymers. These polymers are then processed to make various kinds of plastic products. During polymerization, smaller molecules, called monomers or building blocks, are chemically combined to create larger molecules or a macromolecule.
Polymerization is the process in which the basic units of monomers combine together to form long-chain polymers. Monomers can be either the same or different molecules. Various types of polymers like PVC, Bakelite, and Teflon can be produced by different polymerization techniques.
Polymerization is a process through which a large number of monomer molecules react together to form a polymer. The macromolecules produced from a polymerization may have a linear or a branched structure. They can also assume the shape of a complex, three-dimensional network. There exist several categories of polymerization reactions; the most important ones are step-growth polymerization, chain-growth polymerization (both fall under the category of addition polymerization), and condensation polymerization.
A polymer is a substance that is made up of very large molecules that are, in turn, made up of many repeating units called monomers. Polymerization is the process through which these monomers come together to form the macromolecules that constitute polymers. An illustration detailing the polymerization of the monomer styrene into the polymer known as polystyrene is provided below.
Depending on the functional groups present in the reacting monomers, the complexity of the mechanism of the polymerization reaction may vary. The most simple polymerization reactions involve the formation of polymers from alkenes via free-radical reaction. Polyethylene, which is one of the most commercially important polymers, is prepared through such a polymerization process (the reactant monomer used here is ethylene).
It should be noted that polymerizations involving only one type of monomer are called photo-polymerization, whereas those involving more than one type of monomer are called copolymerization processes. In simple words, we can describe polymerization as a chemical process that results in the formation of polymers or the process of creating polymers. When polymerization occurs, the smaller molecules, which are known as monomers via chemical reaction, are combined to form larger molecules. A collection of these large molecules form a polymer. The term polymer, in general, means “large molecules” with higher molecular mass. They are also referred to as macromolecules.
2. Condensation Polymerization
3. Chain Growth Polymerization
4. Preparation of Polymers
5. Classification of Polymerization
Polymers are formed by the addition of a network of structural units or monomers, as mentioned above. The interesting part is that these are reactive molecules and are usually linked to each other by covalent bonds. These monomers add together to form a long chain to form a product with specific properties. This whole process of the formation of polymers is called polymerization. Polythene and Nylon 66 are some examples of polymers.
Generally, polymerization consists of three steps which include initiation, propagation, and termination. As for the reaction mechanism, the process of polymerization mainly involves two different methods, the step-growth mechanism and the chain-growth mechanism.
In step-growth polymerization, the polymers are formed by the independent reaction between the functional groups of simple monomer units. In step-growth, each step may consist of a combination of two polymers having a different or the same length to form a longer-length molecule.
The reaction is a lengthy process, and the molecular mass is increased at a very slow rate. An example of step-growth polymerization is condensation polymerization, where a water molecule is evolved in the reaction when the chain is lengthened.
Condensation Polymerization
In condensation polymerization, the formation of the polymer occurs when there is a loss of some small molecules as byproducts through the reaction, where molecules are joined together. The byproducts formed may be water or hydrogen chloride. Polyamide and proteins are examples of condensation polymers.
Some of the different types of condensation polymerization are given below.
Polyamides
They are synthetic fibres and are called nylons. These polymers have an amide linkage between them. Condensation polymerization of di-amines with di-carboxylic acid and also of amino acids and their lactams will create a polyamide.
a. Nylon 66: This polymer is prepared under the condition of high pressure and temperature by the condensation polymerization of hexamethylenediamine with adipic acid.
b. Nylon 6: It is prepared by heating caprolactam with water under high temperatures. It is used for tyre cords, fabrics and ropes.
Polyesters
When dicarboxylic acids and diols undergo poly-condensation, polyesters are formed. They are prepared by heating a mixture of terephthalic acid and ethylene glycol at 460 k by using zinc acetate antimony trioxide as a catalyst. Dacron or terylene are the best-known examples of polyesters. And also they are used for glass reinforcing materials in safety helmets.
Phenol-Formaldehyde Polymer
These are the old synthetic polymers obtained by condensation polymerization of phenol with formaldehyde in the presence of either an acid or base as a catalyst.
Novolac, on heating with formaldehyde, undergoes crosslinking and forms an infusible sold mass named as Bakelite. They are used for combs, electric switches and phonograph records.
Melamine-Formaldehyde Polymer
It is formed by the condensation polymerization of melamine and formaldehyde in certain conditions. They are used for the manufacture of unbreakable crockery.
In chain-growth polymerization, the molecules of the monomers are added together to form a large chain. The monomers added may be the same type or different. Generally, alkenes, alkadienes and their derivatives are used. In this mode, the lengthening of chains occurs as a result of the formation of either free radicals or ionic species.
Many of the monomers, like alkenes or dienes and their derivatives, are polymerized in the presence of free radicals. The polymerization of ethene to polythene is by heating or exposing it to light by using a small amount of benzoyl peroxide initiator. The phenyl free radical formed by peroxide is added to the ethene double bond and hence forms a new larger free radical.
It is called a chain initiation step. This newly formed radical will react with another molecule of ethene to form another new free radical, and so on. This repeated formation of a new free radical is called chain propagation. Finally, at some stage, the polymerized product will be formed, and this step is called a chain termination step. The steps are detailed below.
The three steps followed by a free radical mechanism:
When we talk about polymerization chemical reactions, we basically refer to a polymerization reaction of organic monomers. These monomers are in a solution, which further consists of particles that will be coated with the formed polymer deposited on the particle surface. This leads to the formation of a coating layer. The reaction includes either monomer adsorption polymerization or emulsion polymerization.
There are two types of polyethylenes, and they are given below:
1. Low-Density Polyethene
This type of polymer is obtained by the polymerization of ethene under the condition of high pressure of 1000 to 2000 atmospheres at 350 to 520 k temperature in the presence of dioxygen or peroxide initiator as a catalyst in a very small amount.
It is formed through the free radical addition and H-atom abstraction, having a highly branched structure. It is chemically inert in nature and tough but flexible. It is a poor conductor of electricity. LDP is used for the manufacture of toys, squeeze bottles and flexible pipes.
2. High-Density Polyethene
It is prepared by the polymerization addition of ethene in the presence of a catalyst like triethyl aluminium and titanium tetrachloride. The process takes place in a hydrocarbon solvent, in a condition of low pressure of 3 to 4 atmospheres and 343 k temperature.
Like the LDP, it is chemically inert but comparatively tougher and harder. It is used for the manufacture of buckets, dustbins, pipes, etc.
It is also known as Teflon and is manufactured by heating tetrafluoroethylene with a free radical at high pressure. Teflon is chemically inert and less corrosive due to its resistive property against corrosive agents. It is used for gaskets and non-stick surface-coated utensils.
This polymer is formed by the addition polymerization of acrylonitrile in the presence of a peroxide catalyst. It is used as a substitute for wool in the making of commercial fibres such as Acrilan.
It is an addition polymerization that involves the polymerization of monomers that are initiated with anions. This polymerization will be initiated by the transfer of electrons from the ion to the monomer.
The initiators used may be weakly nucleophilic if the monomer is highly electrophilic. In the propagation, the complete consumption of monomer occurs, and this will be faster even at low temperatures. Generally, vinyl monomers are polymerized by this method. It is very sensitive to the solvent used in the reaction. This method is used for the production of synthetic polydiene rubbers, SBR and thermoplastic styrene elastomers.
Polymers are classified into different categories based on several factors such as source, structures, mode of polymerization, molecular forces and growth of polymers. Let’s discuss them in detail below.
Polymers are again divided into three subcategories:
1. Natural polymers: They are found naturally in plants and animals. Resins, starch and rubber are examples of this.
2. Semi-synthetic polymers: This is a modified version of natural rubber; rubbers are treated with chemicals to make them semi-synthetic. Cellulose acetate and cellulose nitrate are examples that come under this subcategory.
3. Synthetic polymers: Polymers which are completely man-made are called synthetic polymers. Polythene, Nylon 66, and synthetic rubber are the widely used synthetic polymers.
There are three different types polymers, based on their structure:
1. Linear polymers: They consist of a long and straight-chain of monomers. PVC is a linear polymer
2. Branched polymers: They are linear polymers containing some branches. Low-density polythene is an example.
3. Network or cross-linked polymer: Polymers having cross-linked bonds with each other is called cross-linked or network polymer. Generally, they are formed from bi-functional or tri-functional monomers. Bakelite and melamine are examples of this type of polymer.
Based on the mode of polymerization, they are divided into two subcategories:
1. Addition polymers: Polymers formed by the repeated addition of monomers by possessing double or triple bonds are called addition polymers. If the addition is of the same species, they are called homo-polymers, and if the addition is of different monomers, they are called copolymers. Examples are polythene and Buna-s, respectively.
2. Condensation polymers: These polymers are formed by repeated condensation of tri or bi functional monomeric units. In this reaction, the elimination of some small molecules, like water and hydrogen chloride etc., will take place. Terylene and Nylon 6.6 are examples.
They are again classified into four subgroups.
1. Elastomers: Polymers that are rubber-like solids and have elastic properties. Here, the polymer bonds are held together by weak intermolecular forces and that allows these polymers to stretch. The cross-links present in the polymer between the chains help to retrace the original position after the removal of the applied force. Examples are Buna-s and Buna-s.
2. Fibres: They are polymers having strong intermolecular forces like hydrogen bonding. Due to this strong force, molecules are kept closer, that is, they are closely packed. Because of this property, they are crystalline in nature. Polyamide and polyesters are examples.
3. Thermoplastic polymers: These are the liner or slightly changed to branched polymers that can be softened on continuous heating and hardened on cooling. Their intermolecular force lies in between the fibres and elastomers. Polyvinyls, polystyrene etc., are examples of thermoplastic polymers.
4. Thermosetting polymers: These polymers come under the category of heavily branched or cross-linked, which can mould on heating and can’t regain the original shape. So, these cannot be reused. Bakelite is an example.
Rubber: Rubber is a type of material called a polymer. It can be produced from natural sources (e.g. natural rubber) or can be synthesised on an industrial scale. Many things are made from rubber, like gloves, tires, plugs, and masks. Some things can be made only from rubber. Sometimes the word means only natural rubber (latex rubber). Natural rubber is made from the white sap of some trees such as the Hevea brasiliensis (Euphorbiaceae). Synthetic rubbers are made by chemical processes.
Synthetic Rubber: In the 20th century, synthetic (artificial) rubbers such as Neoprene began to be used. They were much used when World War II cut off supplies of natural rubber. They have continued to grow because natural rubber is becoming scarce and also because for some uses they are better than natural rubber.
Uses of Rubber: Rubber moulded products are widely used industrially (and in some household applications) in the form of rubber goods and appliances. Rubber is used in garden hoses and pipes for small scale gardening applications. Most of the tyres and tubes used in automobiles are made up of rubber. Rubber plays a very important role in the automobile industry and the transportation industry. Rubber products are also employed in matting and flooring applications.
Difference between Polymer and rubber: Rubber is a natural polymer of isoprene (polyisoprene), and an elastomer (a stretchy polymer). Polymers are simply chains of molecules that can be linked together. Rubber is one of the few naturally occurring polymers and prized for its high stretch ratio, resilience, and water-proof properties. Other examples of natural polymers include tortoise shell, amber, and animal horn.[38] When harvested, latex rubber takes the form of latex, an opaque, white, milky suspension of rubber particles in water. It is then transformed through industrial processes to the common solid form so commonly seen today.
Here, we will list some of the important uses of polymers in our everyday life.
a. Polypropene finds usage in a broad range of industries, such as textiles, packaging, stationery, plastics, aircraft, construction, rope, toys, etc.
b. Polystyrene is one of the most common plastic actively used in the packaging industry. Bottles, toys, containers, trays, disposable glasses and plates, TV cabinets and lids are some of the daily-used products made up of polystyrene. It is also used as an insulator.
c. The most important use of polyvinyl chloride is the manufacture of sewage pipes. It is also used as an insulator in electric cables.
d. Polyvinyl chloride is used in clothing and furniture and has recently become popular for the construction of doors and windows as well. It is also used in vinyl flooring.
e. Urea-formaldehyde resins are used for making adhesives, moulds, laminated sheets, unbreakable containers, etc.
f. Glyptal is used for making paints, coatings and lacquers.
g. Bakelite is used for making electrical switches, kitchen products, toys, jewellery, firearms, insulators, computer discs, etc.
Polymer
Monomer
Uses of Polymer
Rubber
Isoprene (1, 2-methyl 1 – 1, 3-butadiene)
Making tyres, elastic materials
BUNA – S
(a) 1, 3-butadiene (b) Styrene
Synthetic rubber
BUNA – N
(a) 1, 3-butadiene (b) Vinyl Cyanide
Teflon
Tetra Fluoro Ethane
Non-stick cookware – plastics
Terylene
(a) Ethylene glycol (b) Terephthalic acid
Fabric
Glyptal
(a) Ethylene glycol (b) Phthalic acid
Bakelite
(a) Phenol (b) Formaldehyde
Plastic switches, Mugs, buckets
PVC
Vinyl Cyanide
Tubes, Pipes
Melamine Formaldehyde Resin
(a) Melamine (b) Formaldehyde
Ceramic, plastic material
Nylon-6
Caprolactam
In polymers, monomers are bonded by different molecular interactions. The nature of these interactions yields polymers of varying elasticity, tensile strength, toughness, thermal stability, etc.
1. Monomers form a linear chain with weak bonding. These polymers exhibit elasticity and are called elastomers. Examples: neoprene, Buna-S and Buna-R.
2. Polymers with strong forces of interaction between the monomer in both linear and between the chains have higher tensile strength and are used as fibres. Examples: Polyamides (nylon6,6) and polyesters (terylene).
3. Polymers having their intermolecular force in between the elastomers and fibres are thermoplastics. They can be repeatedly reprocessed without much change in their polymeric properties. Examples: Polythene and polyvinyl.
4. Monomers that undergo heavy branching get fused on heating and cannot be reused or reprocessed. Such materials are thermosetting plastics. Bakelite and Urea-formaldehyde are examples.
Natural rubber is highly elastic to be of poor physical stability. Adding 5% of sulphur enhances the crosslinking of the linear chains and, thus, improves the stiffening of the rubber for an application like vehicle tires.
Column A
Column B
1
Buna -S
a
Ziegler Natta catalyst
2
Nylon 6-6
b
Addition polymerization
3
High-density polyethene
c
Terephthalic acid ethylene glycol
4
Declon
d
Biodegradable polymer
5
Polymer of glycine and aminocaproic acid
e
Fibre
Answer:
Terephthalic acid
These polymers have functional groups found in natural polymers. Example: Poly β-hydroxybutyrate -co-β-hydroxy valerate (PHBV). This can be degraded by bacterial presence.
COLLOIDS
Photo emulsion is colloids compound.
A colloid as a kind of solution in which the size of solute particle is intermediate between those in true solution and suspension.
Ex- starch solution, milk, blood etc.
Where 1A degree = 10-8 m.
Properties:
1. Heterogeneous is nature.
2. Do not settle down.
3. Filterability (colloid particle will note be separated by filter paper).
4. Mechanical properties:-
a. Brownian movement.
b. Diffusion (high concentration – low concentration).
c. Sedimentation (do not settle down).
5. Colour of the solution (will depend on size and shape).
6. Electrical properties.
7. Absorption (due to high molecular weight on the surface of colloid partial the other particles which are suspended in the solution will be accumulated).
8. Optical properties.
Types of solution: basis of size of particle dispersed size.
1. True solution: it is “homogeneous” solution containing dispersed particles of molecular size. The particles of solute are invisible.
2. Suspension: it is a “heterogeneous” mixture containing suspended insoluble particles of size greater than 1000A or 100nm. It particles are cannot pass through an ordinary filter paper.
3. Colloidal solution: it is a “heterogeneous” two phase system in which a substance is distribution in colloidal state in an insoluble medium. The particles of the dispersed substance in internal or discontinuous phase, are called dispersed phase, while insoluble medium or external phase, in which they are dispersed, is called dispersion medium.
Suspersion medium
water
alcohol
benezene
air
Same of colloidal solution
hydrosol
alcosol
benzosol
aerosol
Types of colloidal solution: based on their solvent affinity.
1. Lyophilic: solutions are those in which the dispersion medium possesses great affinity for the dispered phase.
Ex: starch, gelatine, glue, agar solutions in water.
2. Lyophobic: solutions are those in which there is no apparent affinity/ interaction between the dispersion medium add the dispersed phase.
Ex: gold, silver solution, and arsenic sulphite solution in water.
Multi molecular, macromolecular and associates colloids:
1. Multi molecular: it consist of aggregates of atoms or molecular having diameter less than 1mm.
Ex: gold solution in water consists of dispersed particles of various made up of “several atoms of gold”.
2. Macromolecular: it particles are very large molecular of high molecular mass.
Ex: starch, cellulose, proteins, natural rubber, synthetic polymer (polythene, nylon, synthetic rubber, polystyrene etc.), egg albumin.
3 Associated colloids: it particles behave as normal strong electrolyles at low concentration but at higher concentration.
Ex: soaps and synthetic detergents.
Characteristics of colloids solution:
1. Heterogeneous nature: it consisting of two distinct phases (the dispersed phase and the dispersion medium).
2. Filterability: colloidal particles can readily pass through ordinary filer paper.
3. Colour: the size and shape of dispersed particles, affect the colour of the solution.
Ex: spherical gold particles impart a red colour to gold solution; while flat particles a blue colour.
4. Adsorption: the colloidal particles, due to the presence of unbalanced forces on their surface, attach a variety of suspended particles of their surfaces.
5. Mechanical properties:
a. Brownian movement: this motion is rapid in case of particles of smaller sizes and in less viscous dispersion medium.
b. Diffusion: the diffusion process has been used to separate colloidal particles of different sizes and to determine their sizes.
c. Sedimentation.
6. Optical properties or tyndall effect: if a powerful beam of light is passed through a colloid solution (contained in a glass cell) placed in a dark room the path of the beam become visible when viewing through a microscope placed at right angle to the path of light. Due to scattering of light by the colloid particles the tyndall effect is happened. The molecule of true solution do not scatter light as their size comparatively very small.
7. Electrical properties: colloidal particles are electrically charged either positive or negative. When a high potential gradient is applied between U tube filled particle with a colloid solution and rest with distilled water the colloid particles move towards oppositely charge electrode. On reaching the electrode they lose their movement of colloid particles under the influence of an electric field known as electrophoresis.
Application of colloids:
1. In everyday life: the food (milk, butter, cheese, fruit etc.) the clothes and shoes that we wear are based on colloids.
2. In analytical chemistry: silica and alumina gels are used as absorbent for gases and as drying agents in laboratory.
3. In medicine: argyrols and protargrol are colloidal solutions of Ag and used as eye-lotions.
4. In industry: smoke precipitation, purification of water, leather tanning, in laundry.
5. In nature: the blue of sky, tails of comets etc. are due to scattering of upon by the colloidal particles of dust or smoke in air.
Light sensitive coating used for plate making are of colloidal in nature important are dichromated colloids, each containing of dichromatic salt combined with natural or synthetic polymer e.g. potassium dichromate and gelation, ammonium dichromate and PVA.
Colloids used in printing:
Negative working colloids and form hand coating.
1. Egg albumin
2. Casein
3. Soya protein
Positive working colloids forms partially hard coating.
1. Gum Arabic
2. Polyvinyl alcohol
3. Fish glue
Ammonium dichromate solution and colloids are coated on plate, exposed and become insoluble in water to a degree. This happened appeals due to oxidation.
Colloids in combination of chromo salt and this material used in plate making depending nature of the electronegative or electropositive. It is used for different type of plate making like surface plate, other paper based plate deep etch paper etc.
1. Egg albumin: based on protein material on ammonium dichromate is mixed with optical sensitization. It used in letter press block and surface making, preservatives are added for stability.
2. Casein:
3. Soya protein:
4. Gum Arabic: gum Arabic which obtained from acacia tree and supplied in form of yellow brown lump. It is dissolved in water to for gumming dispersion. It is used with ammonium dichromate in deep etch plate making.
5. PVA:
6. Fish glue: it is obtained from fish lain, bond. It is in mixed with ammonium dichromate for preparing coating solution. Earlier it was used in halftone block for letter press printing.
7. Gelatine: it is a protein based material and mixed with potassium dichromate from plate making and gravure cylinder making.
Paper Manufacturing:
Fibrous and non-fibrous raw material used in paper and board: two material used while paper make.
1. fibers material
2. non-fibrous material
Fibrous: cellulose fibre can be regarded as the common building brick of the tree. In a few material like cotton and linen, the cellulose exists in a purer form.
Fibres have the form of long usually hollow tubes. This range in length from 1 to 7.50mm and width from 0.01 to 0.05mm according to the plant in which they occur. Cellulose made of carbon, hydrogen and oxygen.
1. Cotton fibres: it is second hair of the cotton plant of natural cellulose. It is used in high grade writing, currency and legal paper.
2. Linen: it is obtained from the ring of last tissues of the stem of the flax plant but as with cotton. It is used in thin strong paper like bank notes and air mill paper.
3. Wood: wood pulp is spruce, pine, deciduous, aspen, eucalytus, birch are some of the material are mix and clean and make white bulky opaque and uniform sheet for printing paper.
A pure form of wood cutting and subjecting it to chemical treatment and remove the natural gums and resin leaving a soft pure pulp.
It is made of mechanical pulp in found dust and doing grinding process by left impurities and make a less strength and poor color paper.
4. Sparto grass: it is found North Africa and Spain. It is made good writing paper.
5. Straw: it is stem of the wheat, it is made writing paper like bank and bond paper and high bursting strength.
6. Manila: it is found from Philipine Island and used in wrapping tissue Cigrette paper and bank note.
7. Jute: it is tissues of annual Indian plant and shoter than linen and hamp used in the thin wrapping.
8. Ramie and China-grass: it is used in Bible paper.
9. Bagasse: it is obtain from sugarcane and used in corrugated paper and board.
Categories of fibres
1. Animal fibre: leather and silk.
2. Vegetable:
A. Stem: wood (softwood, hardwood), Basts (flax, hemp, jute), grasses (esparto, straw, bagasse).
B. Leaf fibre: manila
C. Fruit fibre: seed hair (cotton).
D. Wood:-
a. Softwood: (coniferous tree) softwood is pine and it is made long fibres and strength paper.
b. Hardwood: hardwood is beech and made is short fibres, good bulk, opacity, softness.
3. Mineral fibres: glass, mica.
4. Man-made: nylon, rayon, synthetics.
Paper
Wood pulp
Wood free pulp
1. It is softwood and hardwood pulp
1. The strongest and best quality form
2. Used in newspaper.
2. Cotton, linen, jute or other cellulose fibres.
Non-fibrous material: like chemicals
1. Filler and loader: it is used for strength and optical agent.
a. Strength: (tensile strength, bursting strength, tearing strength)
b. Optical agent: (brightness, opacity, reflectance).
2. Sizing agent
Manufacturing of paper: Preparation of pulp: Three method use in making pulp
1. Mechanical pulping:
a. The tree cut into logs and grinding in big revolving stone and steam.
b. This process during water mix fibres convert into pulp form.
In this process by made paper is good absorbency, bulking and opacity but it become yellow and brittle caused by obtained resins, gums and lignin content
2. Mechanical/chemical pulping:
a. Initial preparation: wood cut into small pieces.
b. Refiner mechanical: a series of disc presence of water called breaking.
c. Thermo-mechanical: digesting, it is cooking material under pressure in 135 degree celcious and it process commonly used in new paper.
d. Chemical-thermo-mechanical pulp: washing, bleaching, and screening.
1. The pulp is washed in clean water and leaker are removed.
2. Bleaching solution chloride is remove coloring impurities.
3. Screening is remove fibre lump and knots.
e. Bio-chi-thermo-mechanical pulp: it also called beating and it is decrease opacity, bulk and strength and increase transparency or basts and tensile strength.
Give more strength and fibres hold together for mix aceater 7%.
3. Chemical pulping: it is obtain wood and other vegetable raw material. It is aim reduce the lignins because the separate more cleanly from each other.
a. Initial preparation: same as above
b. Breaking: in this process sheet and pulp mix with water and some ingredients will be added.
1. Sizing agent: it is used for resistance to wetting and penetration and it is sticky substance and bind fibres together.
Sizing material are:
a. Resin size:- aluminum sulphate add with paper
b. Paraffin wax
c. Wax derivatives
d. Asphalt emulsion
e. Synthetic sizing agent.
The principal method of sizing:
1. Engine sizing: rosin add for normal size.
2. Surface sizing: starch used when paper made in machine and other used PVA, gelatin and it is water resist and prevent ink from ‘feathering’. When the paper is printed.
Reduce fluffing.
3. Tub sizing: it is used in reel for pass in two roller easily.
Grade of sizing paper
1. Unsized: blotting paper
2. Slack size: fast ink penetration, newspaper.
3. Medium size: speedy drying uncoated book stock.
4. Hard size: offset litho cartridge papers.
2. Fillers and loading: loading and fillers and coloring material.
a. It is main purpose is to fill-up gap to it the fibres and improve opacity and form a smooth and uniform surface.
b. Printing paper: 15%-25% used.
c. Bond and ledger paper: 2%-6% used.
Ingredients used:
1. China clay: smooth surface and ink drying.
2. Calcium carbonate: for hardness, opacity and whiteness, increasingly.
3. Titanium dioxide: it is important pigment and growing whiteness.
D. Digesting: (1) sulfate, (2) sulphite
1. Sulfate: it is kraft process and used for soft wood and hardwood
85% chemical pulp produced in world wide.
Caustic soda (soda hydroxide), sodium sulphide (Na2s) and cooked after 2-3 hours fibres separate easily.
2. Sulphite: calcium bisulfite and sulfurous acid in water.
(Green leaker, white leaker, black leaker)
e. Bleaching: brighten, whiten, purity.
Ingredient: chlorine and oxygen and hydrogen per oxide.
f. Beating: fibres made more flexible and bonding.
Varieties of paper:
1. Newspaper print-absorbing ability, quick drying, 75% mechanical wood pulp only coarse-screen halftone, good bulk and opacity lacks strength, yellow and brittle color.
2. Mechanical printing paper: chemical wood pulp writing, paper, one side smooth and thinner.
3. Machine finished: high calendering.
4. Super calendered: suitable for 100 lines upto screen rulling, extra polish and smooth.
5. Wood-free-chemical wood pulp, good color for magazine, leaflets, Booklets, reports 135 gsm.
6. Bible paper: thin white opaque paper, it is made cotton or linen with sulphite, Tio2, china clay.
7. Antique paper: esparto sulphite & soda with up to 15% china clay, rough surface short fibre. It is available 70 to 90 gsm, it is not suitable for writing purpose.
8. Catridge paper: the surface is uneven but strong use for drawing and painting and screen printing. It is not use for halftone and multicolor.
9. Offset cartridge paper: non-fluffy paper, esparto, sulphite or soda with china clay, Tio2.
10. Art paper: fine halftone, art books, technical journal, pamphelts.
11. Chromo paper: one side coated, printing of labels, stickers, posters, book jackets.
12. Cast coated: paper have mirror like glossy, it is only used in USA.
13. Imitation art: not coated, writing paper.
14. Bond paper: matt surface, more opaque, not penetrate ink easily used for letter heading.
15. Ledger paper: strong and durable writing paper made rage and cellulose additives used in banks, it is light blue, light green, yellow, off-white.
Kinds or board
1. Pulp board: pulp board is manufactured in a single-web, wood-free (200 to 750 microns)
2. Triplex board: triplex board is made up of three layers white duplex board consists of two piles or webs.
3. Straw board: book binding, yellow-brownish.
4. Carton board: multi-ply board, test all quality control.
5. Chip board: some straw but more smoother, flexible, making boxes, cartons, containers.
6. Mill board: mill board used for fine leather binding, library work, making split board.
7. Paper board: one side increase printability. It is used for carton, two type board
a. Bending
b. Non-bending board.
Testing
Parameters
Acceptance criteria
Measuring mode
Grammage
+/- 5%
weighing scale
Stiffness MD (machine direction)
+/-10%
stiffness tester
Stiffness CD (cross direction)
+/- 10%
1. Paper:
Aim: check out GSM of paper according to requirement.
Sampling: draw out sample randomly as per sampling plan for paper and paper board.
Check sheets of 1-2 packets/pallets visually for loose particles, specks, spots, smoothness of top and back surface etc.
Test procedure: testing of paper and paper board should be done as per following.
Grammage: grammage is defined as the weight/mass per unit surface area of paper board. The S.I metric unit in which grammage is expressed in Grams/square meter (GSM).
Significance: most of paper is sold in accordance with its grammage and therefore grammage has great significance both to the cunsumer and the procedure in defining price. The value of many physical properties such as bursting strength, thickness and bulk interpreted and specified with regarded to grammage.
Apparatus: a weighing scale. It is a special sheet weighing device designed to weight test speciman of 500 square cm (i.e. 20cmX25cm) area current, when it is in use.
1. Template for preparing test speciman (20X25cm size).
2. A sharp cutting.
3. A hard surface such for cutting the sample.
Sampling & test speciman:
1. Sampling to be done as per sampling plan shown as above.
2. Cut atleast 8-10 pieces of 25cmX20cm size from test speciman (1 each from no. of sample board taken) with the help of the template.
Procedure: check grammage one by one on the weighting scale.
Formula in CM: weight X 10000 / length X width.
Weight: 1.67 X 10000 / 10 X 10
Weight = 167 gms.
Formula in Meter: weight / length X width.
Report: report the grammage as arithmetic means of the value of the test speciman cut in grams/square meter. The range of maximum to minimum (i.e specification) is also to be reported.
Acceptance creteria: it the value of grammage tested is not within the standard specification range i.e +/-5%, then the lot is liable to rejection.
However, the lot may be accepted within same condition after taking approval from QA (incharge).
2. Stiffness test: it is unit milineuter meter (mnm).
Aim: to check for resist towards bending of paper.
Sampling: draw out flat surface samples form the sheets and cut the required MD & CD sample form that sheet by cutter.
Appratus: stiffness tester, weight box, cutter.
Sample size: length=7cm and width=3.8cm.
Significance: stiffness test is a very important test in board, since the utility of the box depends upon its ability to resists bulging when filled, packages must resists deformation or bulging, when being filled and when the contents settle in a package-folding cartons must also withstand bending stresses from loads imposed on them from containers stacked above.
Leveling: before starting the machine please ensure that all three pointers of pendulum, driving disc and stationery disc and zero. Leveling can be done with the help of leveling screws at the base of the stand.
Cutting of test strip: cut the test strip with the help cut shear, place the material to be cut at the back guage with an additional quick blow with the ball of the hand to detach strip form the sheet. Test smaple will be cut in 1 ½ “X 2 ¾ “size. Immediately mark the grain direction of the board to identify MD & CD.
Formula: Left X 5 + right X 5 / 2.
Ex: 25X5 + 25X5 / 2.
125 + 125 /2 250/2 125
Operating the instrument: we know strip insert in jaws in marked and down mark according roller only contact not pressure cause pendulum move and the slowly move the roller ¼ turn in reverse direction same procedure R.H.S and put a callibrated range weight on the lower pendulum stud. For 500 unit different range. The mark on the pendulum aligns to the 7 ½ or 15 degree mark on the driving disc. If pointer mark 25 note the reading as 25X5=125 for range weight of 1000 unit the reading will be 25X10=25 taber unit.
Acceptance criteria: stiffness reading should be within 10% of the specified stiffness values.
Plastic:
1. PVC: clear, white, opaque, gloss, matt.
Used: film
Rigid sheet for blister packaging stretched bottles.
2. Polypropylene:
PRINTING INK
Definition: ink is a liquid or paste that contain pigment of dyes and is used to color a surface to produce an image, text and design.
Two types of ink:
1. Paste: letter press, offset, screen
2. Liquid: gravure, flexography.
Ingredient of inks:
1. Colorant: (a) pigment (b) dye
A. Pigment used in paste ink.
B. Dye used in liquid ink
2. Binders: varnish or vehicles which consists of oils, resins or alkyds.
3. Carrier substance: solvents
4. Driers
5. Ink additives or modifier
a. Plasticizers
b. Waxes
c. Wetting agents
d. Anti-set off compounds
e. Shortening compounds
f. Reducer
g. Stiffening agent
h. Anti-skinning agents
1. Pigments: pigments made from various solvent such as water, oil, alcohol, and acid like as (sulphur, silica or china clay can be combined with either soda ash or sulfate salts).
a. Organic: soluble dyes, insoluble dyes.
b. Inorganic: naturally occuring, synthetic.
2. Binder: it is bind of pigment to each other.
Vehicle = varnish and additives.
Varnish + resin + drying oil
Vehicle: vehicle is to carry the pigments to the paper surface and protect the image during life span of the print, in adhesion to the paper surface.
Resin: resins are organic compounds and may be blended or chemically combines with ability to be dissolved in some organic solution or natural drying oil in order to improves drying, hardness, scratch resistance and gloss of ink.
Drying oil: oil from animal, vegetable, linseed oil, olive oil which extracted from the seeds of flex plants. These oil absorb atmosphere oxygen and polymerized.
Non-drying oil: it include mineral oil which are high boiling extraction of petroleum.
3. Dries: it is used to accelerate the printing ink after it is transferred on the substrate. Drying oil dried by adding small amount of driers, they are soaps of such metals as cobalt, maganese, lead, cerium or zircomium.
a. Cobalt: disadvantage: discolor tint and white ink. It is liquid form.
b. Maganese and lead dries: these are prepared by grinding (mg) + (lb) in linseed oil varnish. It is paste form.
c. ‘grapho’ or perborate: oxidizer that furnish oxygen.
4. Ink additives (modifiers):
a. Waxes: it is used in vehicle to impart rub resistance to dried ink film. It is reduce set off, improve water resistance, slow drying, reduce tack and viscosity.
b. Anti-skinning agent: agent drying of the ink in litho ink.
c. Antioxidant: prevent form oxygen.
d. Plasticizers: it is high boiling that is low volatility solvent whose main purpose is impart flexibility otherwise the ink. Make resin softer and flexible.
e. Wetting agent: such as fatty acid and alcohol.
f. Anti-set off: prevent from set-off printing problem.
g. Shortening: reduce ink flying or misting. It is a wax.
h. Reducer: ink to make thin.
Properties of ink
1. Tack: it is stickness of ink. Tack may be ability of ink to act as an adhesive.
2. Viscosity: the speed of at which various liquid flow will vary widely, the resistance of the liquid to the force trying to move them.
3. Tixo-trophy: it is used in lithography ink.
4. Length: it is property associated with the ability of an ink to flow and form long filament.
5. Transparence & opacity: how many light and dark ink.
Ink term:
1. Baking: away from fountain or handing back.
2. Strike through: soaking of an ink through the paper and discoloring the reverse side of it.
3. Show through: ink visible in back side.
Ink drying method: The dryer is used to accelerate the printing ink after it is transferred on to the surface. It assist oxidation drying the small amount of dryer with the ink act as catalyst. Assisting chemical change without alternating itself. In the polymerization of the ink film resulting in solidification of the printing ink.
Dryer are compound of cobalt, MN+, and pb has a much slower drying action. Pb is generally purpose dryer. Pb is also quite dryer action in increase with increase temperature. This ink drying faster in hot than in cold weather.
Ink drying mechanism: once the ink has been apply to a substrate. It must be able to dry change from liquid to solids fills quickly. These are following method.
1. Oxidation polymerization: the drying process where the drying oil (linceed, tung etc.) form a hard finish occurs through a process of auto polymerization. In process the unsaturated oxidation. A long chain to get solidify addition of dryer accelerate drying process.
2. Absorption: this involved penetration of ink in to the substrate and the consequent removal of liquid component from the surface this mechanism only operate on porus material such as paper and board. It depend on the lower surface tension.
3. Precipitation: used in letterpress moisture set ink. If an ink with a vehicle consisting of water insoluble resin dissolved in a water miscrible solvent is printed on to paper, water from the paper and the surrounding air diluted the solvent to the point where it can no longer the resin in solution and this resin is precipitated around the pigment to form a dry film of ink. The amount of water in the paper is sufficient to cause rapid precipitation, a jet of stream may be blown on to the wet print. The solvent used in moisture or stream set ink are invariably glycols since these materials are miscible with water.
The vehicle for this type of ink is one of the substance is called DI-lalicol with carbon chain and hydraulic blue and each end the solvent already dissolve with certain type of resin under normal condition.
4. Evaporation drying: process of evaporation is that escaped of molecules from the surface of the liquid into air and away from the liquid. In a bulk of liquid the molecule attract on another in all the direction. In surface of liquid the force attracting the molecule inward are greatest because they are relating the few vapour molecules outside the liquid. It is resulting attraction of three force that give the liquid its surface free energy or surface tension characteristic. In practice the evaporation process is assist by applying heat energy.
By the addition of enter influence:
a. Ultra violet
b. Electron beam
c. Infrared radiation
d. Microwave
5. Radiation induced drying: the two main consideration with their radiation drying and curing assistance are amount of photon energy associate with radiation and absorption characteristics of the ink film. For the radiation drying system relevant absorption ranges are:
UV: 100-380UM.
Visible: 380-780UM
IR: 780-1000 UM
MW: >1000 Um.
a. UV: it shorter wave length and free radicals initiate chain reaction with vehicle and polymeric and hence ink film get solidify.
b. Infrared drying: shorter wavelength lowers viscosity and promotes absorption into pores stock.
c. Electron beam curing: electron accelerate by high voltage short wave length and high energy.
High energy create radicals on collision in molecules in the binder. No photo initiator required. Curing is instantaneous.
d. Microwave drying: microwave interact with polar molecules.
The polarized electric field of molecules causing the molecules to oscillate with wave. The resulting friction effect cause the medium to heat-up called dielectric heating.
Cold set- in newspaper: blacks are carbon black in a high-boiling minerals oil = 70% with asphaltic material. Aromatic hydrocarbons = 15%. Mineral oil have been replaced with grade = 5% aromatics, of which less than 0.1% are polycyclic aromatic hydrocarbons (PAHS).
Heat-set in newspaper: heat-set web offset inks are designed to produce high gloss printed image magazines, books. They contain lower-boiling minerals oils that are removed (within 1 sec) as the printed roll (web) passes through a hot-air oven. A typical formulation might be organic pigments (15-36 wt %), hard resins (25-35 wt %), soft resins and drying oils (5-15 wt %), mineral oil (boiling point-240-260C), (25-40 wt %), and additives (5-10 wt %).
Power of Hydrogen
pH - pH is the measure of acidity or alkalinity of a solution.
The most fundamental acid-base reaction is the dissociation of water:
H2O H+ + OHIn
this reaction, water breaks apart to form a hydrogen ion (H+) and a hydroxyl ion
(OH-).
1. [H+] is the molar concentration of hydrogen
2. [OH- is the molar concentration of hydroxide
Water actually behaves both like an acid and a base. The acidity or basicity of a
substance is defined most typically by the pH value, defined as below:
pH = -log[H+]
a. Also, pH is measured in the fountain solutions that are used when printing the paper on the press.
b. Measured on a scale of 0 to 14, pH7 being neutral, pH above that is alkaline and
below that is acidic.
c. The measure of the acidity or alkalinity of a material or solution.
d. Maintenance of fountain solution at optimum pH is vital to high-grade, trouble free offset printing.
e. 7 is neutral, below 7 is acid, above 7 is alkaline.
f. Acid allows the highest quality printing - 4.8 to 5.3 (}2) pH is typically a good range for sheetfed printing.
g. The acid side of the table allows gum to adhere to the plate better.
Principle of pH Meter
a. pH meter basically works on the fact that interface of two liquids produces a electric potential which can be measured.
b. In other words when a liquid inside an enclosure made of glass is placed inside a solution other than that liquid, there exists an electrochemical potential between the two liquids.
pH meter Components:
It is basically an electrode consisting of 4 components:
1. A measuring electrode: It generates the voltage used to measure pH of the unknown solution.
2. A Reference Electrode: It is used to provide a stable zero voltage connection to the complete the whole circuit.
3. Preamplifier: It is a signal conditioning device and converts the high impedance pH electrode signal to a low impedance signal.
4. Transmitter or Analyzer: It is used to display the sensor’s electrical signal and consists of a temperature sensor to compensate for the change in temperature.
Working of pH meter:
a. When you dip the two electrodes into the blue test solution, some of the hydrogen ions move toward the outer surface of the glass electrode and replace some of the metal ions inside it, while some of the metal ions move from the glass electrode into the blue solution.
b. This ion-swapping process is called ion exchange, and it's the key to how a glass electrode works.
c. Ion-swapping also takes place on the inside surface of the glass electrode from the orange solution.
d. The two solutions on either side of the glass have different acidity, so a different amount of ion-swapping takes place on the two sides of the glass.
e. This creates a different degree of hydrogen-ion activity on the two surfaces of the glass, which means a different amount of electrical charge builds up on them.
f. This charge difference means a tiny voltage (sometimes called a potential difference, typically a few tens or hundreds of millivolts) appears between the two sides of the glass, which produces a difference in voltage between the silver electrode (5) and the reference electrode (8) that shows up as a measurement on the meter.
Dampening solution that is too acidic has the following effects:
a. The printing layer of the plate is fretted resulting in sharp pointed halftone dots.
b. The useful life of the plate is impaired.
c. Ink drying is delayed. In extreme cases, ink does not dry at all.
The effects of alkaline solutions are:
a. High dot increase
b. Tendency towards scumming and emulgating
c. Inks with metallic pigments will oxydate resulting in blunt quality in printing
2) Conductivity of a Dampening Solution:
a. Conductivity unit = μS/cm
b. Conductivity describes how electricity is conducted through a liquid; impurities in the dampening solution allow conductivity to increase.
c. In water or any solution the degree of conductivity is determined by the amount of minerals and other ions present.
d. Conductivity is measured on a linear scale, which is represented by the inverse of resistance. The units of measure are micromhos.
e. When considering fountain solutions, most conductivities fall in the 1000 to 3000 micromhos range.
f. There are several variables that influence conductivity. Organic solvents such as isopropyl alcohol will reduce the actual conductivity reading.
g. A 25 to 30 percent isopropyl alcohol solution can cut the conductivity in half.
h. Temperature also influences conductivity. As the temperature goes up, the conductivity goes up, as temperature decreases, so does the conductivity.
i. A good rule of thumb, is for every 10° F change in temperature, conductivity will change by 100 micromhos.
j. Conductivity should be determined using a “freshly prepared dampening solution”, so that this measure can then serve as a “benchmark” when the dampening solution is later exchanged.
k. When the conductivity in the dampening solution has climbed by approx. 1000 μs/cm, this should be taken as a signal that it is time to change the dampening solution.
l. In order to guard against printing problems, it is recommended that the dampening solution be renewed every 14 days.
HOW A CONDUCTIVITY METER WORKS
a. To measure conductivity we use a machine called a conductivity meter.a. The actual amount of electricity that a given water solution will conduct changes with how far apart the electrodes are and what temperature the water is.
b. The meter has a probe with two electrodes, usually 1 centimeter apart.
c. The meter is equipped with a probe, usually handheld, for field or on-site measurements.
d. After the probe is placed in the liquid to be measured, the meter applies voltage between two electrodes inside the probe.
e. Electrical resistance from the solution causes a drop in voltage, which is read by the meter.
f. The meter converts this reading to milli- or micromhos or milli- or microSiemens per centimeter.
g. This value indicates the total dissolved solids. Total dissolved solids is the amount of solids that can pass through a glass-fiber filter.
Requirement of dampening/ fountain solution.
1. It separates image from non-Image area.
2. It keep the plate clean (desensitize).
3. It protect the plate.
4. Ph value should be stabilize (4.5-5.5).
5. Temperature control.
6. Conductivity: good conductor of electricity.
7. Preservatives are added to maintain the equality for long time.
8. Drying simulator: helps in ink drying.
9. Anti-microbe substance are added to prevent algae, bacteria, fungi etc.
10. Anti-foaming agent: prevent foam fountain solution should be minimum and even.
11. We have to apply a heavy film of water, if we use running water because surface tension of water is very high 72.8.
12. Water hardness: 8-12dh (German hardness) (degree of German hardness).
Adhesive
Adhesives – Paste, Glue, Synthetic Adhesive, Hot-melt, Gum
Paste: Paste is prepared from a mixture of plain flour, water, alum and formaldehyde. It is well mixed and heated on a pot until it is thickened. Formaldehyde is added as disinfectant. Paste is not water resistant, gets affected by humidity, bacteria, fungi and insects.
Glue: Glue is prepared from the bones and skins of the animals by boiling with water. The first output is good quality, transparent “pearl glue”. The second output is “flexible glue”. The final output is of inferior quality, having bad smell and brown in color called as “scotch glue”. Glue named “cassava” has been recently introduced, which can be mixed with cold water and used.
Synthetic Adhesive: These are made from (PVA) Poly Vinyl Acetate and are in white liquid form. These have good tackiness and high flexibility.
Hot melt: It is another synthetic adhesive made from copolymers, resins and waxes. It is 100% solid and is brought to working condition by melting is at 1600 to 1900 C.
Gum: It is obtained from tree. It is thin, nearly in liquid form with low viscosity. It is specifically used in manufacturing of envelopes, labels, stickers etc.
Varnishing
Varnishing is a transparent, hard, protective finish or film that is applied to a printed surface to add a clear glossy attractive appearance. Varnish is available in matte, dull or gloss and can be applied on the entire press sheet or in selected areas.
Full Sheet Varnishing: If the entire surface of the sheet or board is varnished, then it is called as full sheet varnishing.
Spot or Patch Varnishing: If the required area (or) the printed area is applied with varnish, then it is called as spot or patch varnishing.
Gloss varnish: Gloss varnish brings out the colors in a printing. Gloss varnish is normally used as a spot varnish to highlight images or photographs printed on an uncoated paper.
Matt varnish: Matt varnish is normally preferred to avoid glair due to reflection but the colors appear duller in a matt varnish.
Aqueous varnish: Low cost water based aqueous coatings are most commonly used today. They provide good protection from finger prints and other blemishes. Like varnishes aqueous coatings are applied in-line on the press but the aqueous coatings are shiner and smoother than varnish. They also have higher absorption and rub resistance. They are less likely to yellow and environment friendly. They dry faster than varnish.
Solvent based Varnish: Solvent-based varnishes are based on synthetic resins dissolved in petroleum solvents. After application the film-forming substances in varnishes harden due to evaporation of solvent.
UV (Ultra Violet) Coating: Extremely high gloss UV, or Ultra Violet, coating offers more protection than varnish and aqueous coating. UV varnishing is applied as liquid, using a roller, a screen or a blanket and then exposed to UV light. The UV light polymerizes and hardens the coating. UV varnish is glossier than all other varnishes. It offers best rub protection but it is costlier than all other varnishes.
Special Effect Varnishes: The use of varnishes for special effects is not limited merely to surface effects. Unusual designs can be achieved with inked varnishes. The best examples for special effect varnishing are pearlescent varnishing, metallic varnishing etc.
With the help of screen printing we can apply thick coating of varnish. We can feel the relief of the coating with our fingers. Special relief codes, such as Braille lettering or security features in security papers, can be created in this way. Embossed wallpapers are prepared using special effect varnishes with relief effect.
Apart from visual effects, special effect varnish can be used to stimulate the sense of smell. Scented varnishes are used for this purpose. Micro-encapsulated fragrances are embedded in the varnish. When the printed varnish is rubbed, the micro capsules burst and the fragrance is released.