Views: 0 Author: Site Editor Publish Time: 2022-12-09 Origin: Site
Rubber has unique physical and chemical properties. The stress-strain behavior of rubber exhibits the Mullins and Payne effects and is often modeled as hyperelastic. The rubber strain crystallizes. Natural rubber is sensitive to vulcanization and susceptible to ozone cracking due to the weak allyl C-H bond in each repeat unit.The two main solvents for rubber are turpentine and naphtha (petroleum).Since the rubber is not easily soluble,the material is finely divided by chopping before dipping.Ammonia solution can be used to prevent coagulation of raw latex.Rubber starts to melt at about 180 °C (356 °F).
On a microscopic scale,loose rubber is a messy,irregularly changing wrinkle chain.In stretch rubber,the chain is almost linear.The restorative force is due to the predominance of wrinkled architecture over a more linear architecture.See ideal chains for a quantitative treatment,and entropic forces for more examples.Cooling below the glass transition temperature allows local conformational changes,but reordering is practically impossible because of the larger energy barrier for coordinated motion of longer chains."Frozen" rubber has so little elasticity that small changes in bond length and bond angles cause strain: this led to the Challenger disaster,when the flat O-rings on the US space shuttle failed to relax to fill the widening gap.The glass transition is fast and reversible: the force is restored upon heating.Parallel chains of stretched rubber tend to crystallize.This takes some time,as the turns of the twisted strands have to move out of the path of the growing crystallites.For example,crystallization occurs when an inflated toy balloon is found to wither with a relatively large residual volume after a few days.It shrinks where you touch it,because your hand is hot enough to melt the crystal.Vulcanization of the rubber creates disulfide and polysulfide bonds between the chains,which restricts the degrees of freedom and causes the chains to tighten faster for a given strain,increasing the spring constant and making the rubber stiffer and less ductile .
Raw rubber storage depots and rubber processing create a stench that is strong enough to be a source of complaints and protests from nearby residents. Microbial impurities originate from the processing of bulk rubber.These impurities decompose and generate VOCs during storage or thermal degradation. Detection of these compounds using gas chromatography/mass spectrometry (GC/MS) and gas chromatography (GC) showed that they contained sulfur, ammonia,olefins,ketones,esters,hydrogen sulfide,nitrogen,and low molecular weight fatty acids (C2 –C5).When producing latex concentrates from rubber, sulfuric acid is used for coagulation.This produces foul-smelling hydrogen sulfide.The industry can mitigate these unpleasant odors with scrubber systems.
Rubber is the polymer cis-1,4-polyisoprene-molecular weight 100,000 to 1,000,000 Daltons.Typically,a small proportion (up to 5% by dry mass) of other materials such as proteins,fatty acids,resins and inorganic materials (salts) are found in natural rubber.Polyisoprene can also be synthesized,producing what is sometimes called "synthetic natural rubber,"but the synthetic and natural routes are different.Some natural rubber sources,such as gutta-percha,consist of trans-1,4-polyisoprene,a structural isomer with similar properties.Natural rubber is an elastomer and thermoplastic.Once the rubber is vulcanized,it is thermoset.Most rubber in everyday use is vulcanized to the point that it has properties of both; that is,if it is heated and cooled,it degrades but is not destroyed. The final performance of rubber products depends not only on polymers,but also on modifiers and fillers,such as carbon black,ointment,white powder,etc.
Rubber granules are formed in the cytoplasm of specialized gelatinous cells called laticifers in rubber plants.The rubber particles are surrounded by a monolayer phospholipid membrane with the hydrophobic tail facing inward.This membrane allows biosynthetic proteins to be sequestered on the surface of the growing rubber particle,which allows the addition of new monomeric units from outside the biofilm but inside the milk duct.A rubber particle is an enzymatically active entity consisting of three layers of material,rubber particles,biofilm, and free monomer units.The biofilm is tightly anchored to the rubber core by the high negative charge of the double bonds along the rubber polymer backbone.Free monomer units and binding proteins make up the outer layer.The rubber precursor is isopentenyl pyrophosphate,an allyl compound,which is elongated by Mg2+-dependent condensation under the action of rubber transferase. Monomers are added to the pyrophosphate end of the growing polymer.This process displaces terminal high-energy pyrophosphates.This reaction produces a cis polymer.The initial step is catalyzed by prenyltransferase,the three monomers of prenyl pyrophosphate are converted to farnesyl pyrophosphate.Farnesyl pyrophosphate can be combined with rubber transferase to extend new rubbery polymers.The required isopentenyl pyrophosphate is obtained from the mevalonate pathway,which is derived from acetyl-CoA in the cytosol.In plants,isoprene pyrophosphate can also be obtained from the 1-deox-D-xyulose-5-phosphate/2-C-methyl-D-erythritol-4-phosphate pathway within the plasmid.The relative proportions of farnesyl pyrophosphate initiator units and isoprene pyrophosphate extension monomers determine the rate of synthesis of new particles versus the rate of elongation of existing particles.Although rubber is known to be produced by only one enzyme,latex extracts contain many small molecular weight proteins with unknown functions.Protein likely acts as a cofactor as synthesis rate decreases with complete removal.