Processing of Natural Rubber Compounding

Processing of Natural Rubber Compounding:

The manufacture of rubber products from dry natural rubber can be divided into four stages: 

1. Mastication

2. Incorporation 

3. Shaping

4. Vulcanization

The rubber must be masticated to bring it to a suitable consistency (viscosity) to accept the compounding ingredients. These ingredients are incorporated into the masticated rubber, after which the rubber is shaped and vulcanized. The shaping and vulcanization stages may be combined as in transfer or injection molding or separated as in the extrusion and subsequent vulcanization of a tube.

Mastication: Unless the producer has modified NR to a specific processing viscosity, it is very tough, and therefore, requires mastication prior to compounding. 

1. During mastication, the NR molecules are mechanically broken down by means of high shear forces. The mastication can be carried out on mills at low temperatures, or at elevated temperatures in the presence of peptising.

2. High degrees of mastication are only required, if the NR is to be used in very soft compounds, or if the compound is to be dissolved in solvents (sponge, frictioning, rubber solutions). 

3. Low degrees of mastication make it already possible to incorporate chemicals and fillers readily in NR.

Mixing on Mills:

1. The rubber is first worked on the mill until a coherent band is formed on the mill rolls. 

2. On adding softeners, the band will generally split, and it has to heal before additional fillers are added to the compound. Finally, the sulphur is mixed in, if no ultra fast accelerators are present. 

3. The sul­phur is added on the warm up mill before processing. During the mixing process, the band must not be cut, and only after all ingredients have been incorporated in the compound, the band is cut and folded, that is, the compound is homogenized. 

4. When the mixing cycle is completed, the compound is cut from the mill as slabs, and cooled in a water bath and stored, or it is cut into slabs on batch-off equipment.

Mixing in Internal Mixers

1. When mixing is carried out in internal mixers a relatively hard and nervy rubber is required for good and efficient dispersion of compounding ingredients. 

2. The usual mixing temperatures are about 140 to 150°C, and through intensive cooling, this temperature can be reduced to 120 to 130°C for heat-sensitive compounds. 

3. If no means of cooling is available, the mixing tempera­ture can go as high as 180 to 190°C. 

4. Mixing at such high temperatures, also referred to as "hot mixing", has been popular for some time now, since it allows very short mixing cycles. 

5. However, the properties of the compounds and of the vulcanizates are considerably changed in this mixing process. 

Cooling Mills

1. After mixing, the compound is dumped from the internal mixer onto a cooling mill, which could also be equipped with blending bars. 

2. It is then cut into slabs or festooned in batch-off equipment for it to cool down. 

3. In some large-scale operations, the mixed compound is also palletized, cooled, and then stored sulphur.

Vulcanization:

1. NR compounds can be vulcanized in all commonly used processes - hot air, with or without pressure, steam, hot water, press, transfer moulding, injection moulding, roto cure, molten salt bath, hot air tunnel, high frequency radiation, lead cure, etc.

2. Because of the non-polar character of NR, there may be problems with preheating light-colored NR compounds by high frequency radiation 

3. Therefore, one should either add carbon black, or polar compounds, like triethanol amine, polar factices etc. to the compound. 

4. The higher the cure temperatures, the poorer are the mechanical properties of the vul­canizates, and the shorter is the plateaus.

Sulfur Vulcanization

1. Natural rubber can be compounded without fillers to give a vulcanizate with high elongation (600-800%) and high tensile strength (21-28 MPa or 3000-4000 psi). 

Reinforcing fillers such as carbon black, precipitated silicas, and hard days are used to adjust the hardness. 

2. Whiting (calcium carbonate) is the most common nonreinforcing filler. High loadings can be used to dilute the rubber with the minimum loss of softness, elongation, and resilience. 

3. Most natural rubber vulcanizates are cross-linked with sulphur.

Efficient Vulcanizations (EV)

1. EV systems are characterized by low sulfur content combined with a high accelerator concentration. 

2. A sulfur-donating accelerator may be used to further reduce the elemental sulfur content. 

3. Typically, an EV system contains 0.3-1.0 effective parts of sulfur and 2.0-5.0 parts of accel­erator. T

4. The semi-EV system is a compromise between the conventional and EV with sulfur at 1.0-1.8 parts and the accelerators at 1.0-2.5 parts. 

5. The EV systems produce vulcanizates with a higher concentration of monosulfide cross-links that are more thermally stable and more easily protected against oxidation.

Sulfur and Accelerators:

1. Although Natural Rubber (NR) can also be crosslinked with peroxides, or high energy radiation, in practice, sulfur and accelerators are predominantly used. 

2. For lower sulfur concentrations, larger amounts of accelerator are required, in order to keep up the level of cross-link density. 

3. Low sulphur vulcanization systems play an increasingly important role in addition to the conventional cures with about 1.5 to 2.0 phr sulfur. 

4. For optimum levels of mechanical and dynamic properties of vulcanizates with a good heat resistance, low sulfur 0.5-1.5 phr) or so-called "semi-efficient"' (Semi-EV) vulcanizations are used. "Effi­cient" vulcanizations (EV).

Metal Oxides:

Metal oxides are required in a compound to develop the full potential of accelerators. 

1. The main metal oxide is zinc oxide, but other oxides are used at times to achieve specific results, namely magnesium oxide in the presence of acidic compounding ingredients such as factice, based on sulphur monochloride, or lead oxide, to obtain an especially low water absorption of vulcanizates. 

Activators:

1. Many accelerator systems require additional activators, like fatty acids, or salts of fatty acids, namely stearic acid, zinc soaps, or amine stearate. Glycoles or trpthanol amine also serve as activators, the latter primarily in compounds with reinforcing silica fillers. Fatty acids have a lower activity in low Nitrogen NR Grades.

Vulcanization Inhibitors:

1. These are used to prevent premature vulcanization or scorching. 

2. In many instances, a sufficient scorch life is obtained by using either an­ appropriate combination of accelerators, or acidic compounding ingredients. 

3. When these measures still do not give sufficient scorch protection, special inhibitors are used, primarily those based on phthalimide sulfenamides. 

4. They not only delay the onset of cure, but also the time to completion of cure. 

Protective Agents

1. Because it is highly unsaturated, NR has to be compounded with protective agents to achieve a sufficient aging resistance.

2. The level of protection is determined by the chemical nature of the protective agent. 

3. Most effective are aromatic amines, such as p-phenylene diamine derivatives, which not only protect the vulcanizate against oxidative degradation, but also against dynamic fatigue and degradation from ozone and heat. 

4. It is remarkable that some agents such as PAN or PBN impart a good fatigue/resistance to NR, but none or very little to SBR vulcanizates. 

5. Since the most effective protective agents more or less discolor the vulcanizate, less effective ones, like bisphenols, phenols, polymeric hindered phenols have to be used in light-colored vulcanizates.

Fillers:

1. The active fill­ers do not give quite the same amount of reinforcement as in most SRs, but the effi­ciency of the fillers ranks in the same order in both cases. 

2. Reinforcing fillers enhance the already high tensile properties of gum NR, and they improve, in parti­cular, the abrasion and tear resistance. 

3. Less reinforcing fillers, such as N 770 (SRF) or N 990 (MT), and light inactive fillers, such as kaolin, calcium carbonate, barium sulphate, zinc oxide, or magnesium carbonate are used for a number of reasons. These include an improvement of processibility, and the attainment of particular specifications, such as density, colour, or even price. 

4. Depending on their activity, the fillers determine, more or less, the hardness, and they also reduce the rebound elasticity of NR vulcanizates. 

5. With inactive fillers, and especially with zinc oxide and N 990 black, one can formulate filled vulcanizates, which have almost the same elasticity as unfilled gum vulcanizates. 

6. NR compounds require and therefore con­tain considerably less filler than SR compounds. For instance, with highly active fillers, one generally uses up to about 50 phr, and somewhat higher concentrations with non-active fillers.

Oxidation:

1. The oxidation of a natural rubber compound is a complex process involving many reactions, which are influenced, by processing conditions, metal catalysts, temperature, and formulation. 

2. Oxidation proceeds through chain scission of the polymer backbone. 

3. The hydrogens and double bonds are the primary points for attack. Oxidized natural rubber vulcanizates exhibit inferior physical properties including hardness and, at some point, surface hardness. 

Ozone Attack:

1. In the presence of ozone, the surface of natural rubber vulcanizates develops cracks. 

2. The ozone reacts with the olefinic double bond to yield a 1,2,3-trioxlane, which decomposes to a zwitterion plus an aldehyde, cleaving the double bond. 

3. The zwitterion and aldehyde may recombine to the ozonide or the zwitterion can react with another zwitterion to yield a cyclic diperoxide or a polyperoxide. 

4. Ozone attack on natural rubber can be prevented through the use of wax and chemical antiozonants. 

Softeners and Resins:

1. A great number of different materials serve as softeners, the most important ones being mineral oils. 

2. These oils include a wide range of products, from paraffin to aromatic. Animal and vegetable oils are also important softeners or process aids, to include wool grease, fish oil, pine tar, and soya oil. 

3. NR requires lesser amounts of softener than most SR's. Synthetic softeners, which are commonly used with CR (Chloroprene Rubber) or NBR (Nitrile Rubber), play only a minor role in NR compounding. 

4. When selecting softeners for NR, the potential of blooming or migration has to be considered

Resins:

1. While it is important to add resins to compounds from most SRs, so that they can be fabricated, this is not generally necessary with NR compounds. 

2. If an exceptionally good building tack is required, such as compounds for frictioning of textiles, it is advantageous to add rosin, tar, pitch, or other tackifiers to NR com­pounds. 

3. Those tackifiers, which were specially developed for SRs, are of little rele­vance in compounding of NR. 

Applications:

1. Radial tires use a higher proportion of natural rubber because its high green strength, tack, and cohesive properties help maintain the uniformity of the green tire during construction. The low heat build-up of natural rubber is more important in a radial tire where the sidewall requires more flexibility.

2. Tires of trucks and off the road and earthmoving equipment are frequently damaged and large chunks of rubber can be torn out of the tread. 

3. Aircraft tire treads are also commonly made of all natural rubber, as are automobile engine mountings and suspension systems.

4. Adhesives for surgical, masking, and duct tape as well as packaging adhesives are made from natural rubber.

5. Significance include rubber printing rolls, baby bottle nipples, rubber bands and thread, tank linings, and tennis balls, as well as a wide variety of molded and extruded specialties.

Formulations: 

a) Formulation for Tyres 

Formulations: 

b) Formulation for Conveyer belts 

Formulations: 

c) Formulation for rubber Hose 

Polysaccharides:

Polysaccharides are usually subdivided into fiber-forming linear structural polysaccharides (e.g., cellulose, chitin), moderately to strongly branched reserve polysaccharides (e.g., amylose, pectin), and physically cross-linked, gel­forming polysaccharides (e.g., gums, mucopolysaccharides) (G: sakcharon = sugar). Cellulose and starch are amongst the best known polysaccharides 

Celluloses:

Nature converts glucose into cellulosc (in plants) when thousands of glucose molecules are linked together through hydroxyl groups at C1 and C4 in a long chain forming a polyanhydride (giant cellulose molecule) and several water molecules (by a process similar to polycondensation). The following is the structure of the repeat unit in cellulose 

The reason for this is the stiffness of the chains and hydrogen bonding between two -OH groups on adjacent chains. It has the property of forming long threads and for this reason is largely incorporated as raw material in textile industries Cellulose is highly stereo specific macromolecule. 

The melting temperature of crystalline cellulose is far above its decomposition temperature. The fiber forming properties of cellulose can be accounted by its unique conformation.

Starch:

Starches are intimate mixtures of amylose and amylopectin. They are obtained from com, potatos.

1. Starch is a polymer with an a-glucose as the repeat unit and is a widely distributed polysaccharide. This dissimilarity in the structure of starch makes it available as a granular material, with no trace of organized crystalline structure.

2. Amyloses arc practically linear poly [(1-4)-anhydro-D-glucopyranose]s with molecular weights of up to ca. one million. The stable macroconformation of amy­lose is that of a helix. Amylopectins are branched poly [α-(1 -4)-anhydro-D-glucopyranose]s with Christmas tree-like structures. They possess 1 branching unit per 18-27 glucose units; the branching is via 1.6-positions. 

Glycogen: Glycogen is a polysaccharide, found in animals only. It is an important energy storing material in the body and is found mainly in muscle and liver. This is a glucose polymer similar to amylopectin, except that it is more highly branched.

Dextran: In dextrans, chain bonds are mainly in (l~6') positions and branches via ­(1 -4') bonds. Dextrans are obtained by enzymatic polymerization of saccharose. They serve as blood plasma expanders and as columns in chromatography.

Chitin: Chitin is the structural polysaccharide of arthropods (insects, crustacea) where it forms so called exoskeleton, playing a role analogous to the protein collagen in higher animals. It is always associated with calcium carbonate and/or proteins.