Latex Compounding

To improve difference properties of latex, generally emulsions, dispersion and solutions are added as compounding ingredients.

  

Surfactants

Surface active agents are used to stabilize the latex by reducing the surface free energy. The stabilization means delay the coagulation of latex. There are different types of surfactants as follows,

• Anionic surfactants: carboxylates, Sulfonates  
• Cationic surfactants: Acetyl tri methyl ammonium bromide
• Amphoteric surfactants: Water soluble proteins
• Nonionic surfactants: Poly ethylene fatty acids

Property modifiers

• Vulcanizing system: is used to increase the strength. Generally the vulcanizing agent is S (sulfur). Primary activators like ZDC (zinc dithio carbonate) and secondary activators like thiurum, thiosole are also used.

• Viscosity modifiers: are used to control skin thickness.

• Softeners: are used to increase the softness of rubber. Examples are vegetable oils, organic esters, petroleum oils etc.

• Fillers: are used to increase stiffness of rubber. Examples are carbon black, TiO₂, CaCO3, ZnS etc.

• Antioxidants:  there are two types called Staining Antioxidants and non-staining Antioxidants. Examples are staining- Amines, non staining- phenolic compounds  

Auxiliary ingredients

They are coagulants, heat sensitizers, flame retardants, colorants etc. 

The equipment like Ball mills, Attrition mill,   and colloidal mills are used in latex compounding.   

Natural Rubber Latex

Natural rubber (NR) latex is serum of Havea Brazilienzis tree which is commonly used as rubber tree. Its composition is as follows,

Rubber (dry rubber content)	30-40%
Non rubber content	5-6 %
Water content	55-65%

Non rubbers present in NR are sugar, proteins (amino acids), phospholipids, metals etc. Not only that its contain unreacted COO-, NH3+, NH4+ also there. Generally field latex has a PH of 6-7. But with the time PH is reduced as Bacterial activities present. That means H+ percentage is increased. Because of that latex coagulates. To avoid coagulation PH should be increased by adding bases. It is known as preservation of NR  

Eg: 

• Ammonia of PH 8

High ammonia (0.7-0.8 w/w)

Law ammonia (0.2w/w)

• Non-ionic surfactants
• Cationic surfactants

Since dry rubber content (DRC) is law in NR latex or since high water content it is not economically in transporting, storing etc.  So to increase the DRC different concentration methods are used. Commonly used methods are listed below.

• Creaming

If latex is kept in rest rubber rich layer will lie on top of water rich layer due to the density difference

• Centrifuging

Centrifugation is used to separate rubber compounds from water

• Electro decantation

This is a method which rely on the fact that latex particles carry a negative charge so they will migrate towards a positive electrode through a semi permeable membrane

• Evaporation

This method was developed with the means of stabilizers and controlled evaporation because uncontrolled evaporation leads to skin formation in latex.

Characterization of Natural Rubber latex

There are different methods to do characterization of NR as given below.

1. Total Solid Content (TSC): Measure how much of solid are present in latex
2. Dry Rubber Content (DRC): Measure the pure rubber content. This is very important parameter  
3. Alkalinity: Measure the free alkali content of latex
4. KOH number: How much KOH is consumed in neutralizing volatile and non-volatile acids in latex
5. Mechanical Stability Time (MST): Latex is stirred very rapidly (about 13000 rpm) then time taken to stabilization is observed as MST
6. Volatile Fatty Acid number (VFA): Measures volatile fatty acid content in latex

Latex Products

Rubber is used as both liquid (latex rubber) and solid form (dry rubber) for processing. It was described in previous rubber processing post. Latex products are made from latex concentrate of 60% dry rubber content. Dry rubber products are made from raw rubber prepared by coagulation of rubber latex. When we compare latex rubber and dry rubber main drawback is the high water content in latex rubber. But it has many advantages like it need simple machinery, so law energy consumption, simple methods for compound preparation, law environmental pollution etc.

Latex rubber products are classified as follows,

Thin wall products

• Dipped products

Eg: gloves, balloons, swimming caps

• Cast products

Eg: Masks, toys

Cellular products

Eg: Foam mattresses

Threads

Eg: textile products, embroidery, shoes, elastic bands

Coatings 

                Eg: rubber paints

Both natural rubber latex and synthetic lattices are used for manufacturing products. Natural rubber (NR) latex is serum of Hevea Brazilienzis tree. Synthetic rubber (SR) lattices are developed because of high demand and law availability of NR latex. They are largely produced by emulsion polymerization.

  

When we compare NR latex and SR lattices, SR particles are smaller so high interfacial area per unit volume, law distribution of particle size and less spherical in shape.  It has high mechanical stability, but sensitive to coacervating effect of mechanical pressure. Because of they are synthetic has law tendency of undergoing micro-organic attacks. Wet-gel strength (the ability of maintaining the structure without collapsing under vulcanization at higher temperature) is also low in synthetic rubber lattices.  

NR latex will be discussed in future. 

Rubber Compounding

This post continue from previous post Rubber Compounding.

 Fillers

Functions of fillers are reinforcing the rubber, improving hardness of rubber, increasing the volume of rubber for low cost production, facilitating processing of rubber, and enhance age resistance.

Features for selection of fillers

•      Particle size (determines surface area per gram)

•      Surface activity (physio-chemical nature)

•      Persistent structure

•      porosity

Classification of fillers,

Processing aids/ softeners/ plasticizers

Processing aids have many uses like Prevent rubber mass from adhering to hot machinery surfaces during processing, disperse filler uniformly, and modify physical properties such as elasticity, flexibility

•      Softeners for non-polar rubbers

                Petroleum products: bitumen, waxes, petroleum oils

        Vegetable by-products: pine tar, fatty acids and salts, factice             

Synthetic products: reclaimed rubber, partially cross-linked polymers, plastic materials, liquid rubber

•      Softeners for polar rubbers

Ester plasticizers

Protective agents

Antioxidants

Anti-oxidants are used to inhibit rubber ageing due to oxidation reaction, which is accelerated by heat, light and salts of transitional metals. Examples are Amines and their derivatives, phenols and their derivatives.

There are two types of antioxidant,

• React with free radicals in the stage of their formation and inhibit development of the chain reaction
• Decompose hydro-peroxide into products that are inactive for the development of an oxidizing chain

Auxiliary ingredients 

Blowing agents for production of cellular products

Flame retardants (eg: for fire retardants mats)

Antistatic agents (eg: for Conveyor belts, textile rollers, trolley wheels used in hospitals)

Abrasives (eg: for erasers)
Pigments and dyes  

Rubber Compounding

Rubber compounding is important to improve mechanical properties, enhance weatherability and manipulate processing etc. of a rubber product. Compounding ingredients play a major role in rubber compounding. Some of them are explained below.

Peptizer (Chemical Plasticizer)

Peptizer is used mainly for natural rubber to increases the efficiency of mastication due to chemical reaction with O₂ which is affected to the efficiency of mastication, reduces time of mastication of long molecules and softens rubber for shaping etc.

Examples for peptizer are Pepton 2 (Di-o-benzamide-diphenyl disulfide), Renacit 4 (zinc penta chlorophenol), organic mercaptants etc.

 Vulcanizing system

The functions of the vulcanizing system are controls vulcanization, influence processing, modified tensile and other physical properties and affects heat resistance and other ageing characteristics. There are different types of vulcanizing systems as follows,

• Sulfur vulcanizing system
• Sulfur donor vulcanizing system
• Non-sulfur vulcanizing system

The components of a vulcanizing system are,

• Vulcanizing agent which reacts with active sites in the polymer to form cross links
• Accelerator which accelerates the vulcanizing rate

Type	Rate	Examples
Guanidines	slow	Diphenyl-guanidene (DPG)
Thizoles	medium fast	Mercaptobenzothiasole (MBT)
Sulfonamides	Long delayed action	N-cyclohexyl-2 benzothiasole sulfonamide (CBS)
Thiurams	fast	Tetramethyl thiuram disulfide (TMTD)
Carbamates	ultra-fast	zinc diethyl-dithio-carbamate (ZDC)

• Activator which improves acceleration and cross linking potential and it can be organic and inorganic  

Eg: Organic activator -stearic acid, lauric acid

       Inorganic activator - ZnO, MgO

• Retarder which reduces the tendency of premature vulcanization

Eg: Phthalic anhydride, salicylic acid 

Other compounding ingredients like Fillers, Processingaids, Protectiveagents, Auxiliaryingredients   will be discussed. 

Synthetic Rubber Types

          Nowadays there are many different synthetic rubbers existing in the rubber processing industry other than natural rubber as previously mentioned. Among them some of commonly used types are mentioned here.

• Poly Isoprene Rubber (IR)

Dirt content is low compared to the natural rubber and the color is comparatively good because of the low level of impurities. Even though this contains isoprene the curing time is long as absents of natural non rubber accelerators. 

• Styrene Butadiene Rubber (SBR)

It contains 75-77% butadiene and 23-25% styrene. SBR has low strength due to crystallinity of styrene and has low flexibility due to phenol group. Its weather resistance is good since fewer amounts of unsaturated double bonds.

• Isoprene Isobutylene Rubber (IIR)

IIR is mostly used in membranes and tube manufacturing because it has very law gas permeability. Not only that it has good weather resistance, insulation properties, shock absorption properties. It’s curing time also high.  

• Ethylene Propylene Rubber (EPR)

Sulfur(S) vulcanization can’t be done in EPR. But in some cases monomers like EPDM (ethylene propylene diene monomer) is added to perform sulfur vulcanization. EPR is generally used in outdoor applications and shipping industry as it has good weather and heat resistance.

• Acrylonitrile Butadiene Rubber (NBR)

NBR is a polar rubber which is specially used to produce horses, oil seals, printing rollers and customized products etc. It can be blend with PVC to enhance flame retardant properties. S vulcanization also can be done.

 

• Silicon Rubber (SI)

 SI is an inorganic rubber with inorganic backbone. It is mainly used in manufacturing surgical gloves, by pass valves etc. Its weather resistance and thermal stability is very good.   

• Chloroprene Rubber (CR)

Because of its chlorine (Cl) contain monomer it has high flame retardant property. Sulfur vulcanization can’t be done for it.  CR is also polar rubber. It has good resistance for petroleum oils.

Rubber Processing

 Mixing, shaping and curing are the main three stages in rubber processing. Mixing with different compounding ingredient is done to increase chemical and physical properties of the rubber. That also known as rubber compounding. There are three general processes for shaping called molding, extrusion and calendering. after shaping curing is done for improve hardness of rubber product  by forming cross links in rubber chains. That is also called vulcanizing.  

 Now a day’s both natural rubber and different synthetic rubbers are used for rubber processing.

• Natural rubber(NR)

Chemical structure of Natural Rubber

Natural rubber is latex taken from the tree Heavea Brazilianzis

which is generally known as rubber tree. NR has excellent tensile strength and elongation. Its abrasion resistance, tear resistance, fatigue resistance and electrical resistance also high. NR formula has unsaturated double bonds, so its weather resistance is poor. Not only that natural rubber is dissolved in non polar chemicals like oils, petroleum based fuels etc.

Natural rubber is used as both liquid (latex rubber) and solid form (dry rubber) for processing. Some applications of these two types, some processing methods and advantages & disadvantages are summarized below.

	Applications	Processes	Advantages
Latex rubber	Gloves, Balloons	Dipping, tubing	Simple machinery can use,low energy  consumption
Dry rubber	Tires,  o rings, engine mounts	Extrusion, molding	Easy to transport and store

• Synthetic rubbers

Synthetic rubbers are made by polymerisation of different petrolium based monomers. Some of synthetic rubbers are Isoprene rubber(IR), Styrene Butadiene Rubber(SBR), Isoprene Isobutaline Rubber(IIR), Ethelene Propelene Rubber(EPR), Chloroprene Rubber(CR), Silicon Rubber etc.

 Those synthetic rubbers have different properties, so they have different applications and different processing methods. they will be explained in future.