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.