The Basic Rubber Compound

8/13/2019 The Basic Rubber Compound

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The Basic

Rubber

Compound


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Introduction


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Introduction

First development ( Goodyear and Hancock)

Rubber compound is a mixture of a number ofdifferent ingredients

Raw gum elastomer as adhesive

Significant swelling capacity of raw gum.


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The basic compound formula


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Raw

Materials


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Raw gum elastomer

The key ingredient in rubber industry

Many of the properties of the final product dependupon it


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Sulfur

Widely use cross-linking agent.

Results: 1) More dimensional stable

2) Less heat-sensitivity

3) Less swelling4) Less sticking

Cheap, rubber-makers sulfur (particle sizevariation), oil coated.


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Sulfur Rubber-makers sulfur

suitable for vulcanizing rubber

It has a low ash content

Low acidity

Sufficiently fine for adequate dispersion andreaction

High sulfur level- bloom

Blooming occurs if an additive dissolves totally in

the polymer at the processing temperature but is

only partially soluble at ambient temperature


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Sulfur

Highly amorphous form of sulfur, known as insolublesulfur, is available to reduce this problem.

Dispersion in the compound can be more difficult

NO affect on products performance

Aesthetically displeasing

In the uncured compound, bloom can reduce tack

needed in building operations such as

plying up uncured sheets of rubber to obtain thicker sheets


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Zinc oxide and stearic acid

Zinc oxide + stearic acid zinc stearate

Together with sulfur & accelerator, constitute the cure

system for the formulation and speed up the rate at

which sulfur vulcanization occurs

With this curing system, it can be reduced time from

several hours to few minutes


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Accelerators

speeds up the rate of vulcanization

Grouped into several chemical classes

delayed action accelerator (sulfenamides)

short induction accelerator (dithiocarbamate)

o slow cure accelerator (guanidines)

o Fast cure accelerator (thiurams and

dithiocarbamate)

sulfur donor accelerator (TMTD, DTDM)


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Other cross-linking systems

Peroxides

Electron beam curing Miscellaneous


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Peroxides

suitable for curing rubber they do not need unsaturated bonds

Not as popular as sulfur

In the basic rubber compound formulation, thezinc oxide, stearic acid, sulfur and accelerator canall be replaced by a single material, the peroxide

Contact with oxygen (air) should be avoided

during vulcanization These compounds can interact with the peroxide

in peroxide cure systems and thus interferewith cure


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Peroxides

Advantages

improvement in heat aging resistance ofvulcanizate thus lifetime can be extended

upper processing temperature limits can be

pushed up a little

Disadvantages

Reduced mechanical properties

Post curing is sometimes undertaken for peroxide

cured vulcanizates, to complete the cure and

remove unwanted byproducts


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Peroxides

Coagents

cross-link density of a peroxide cured

compound can be increased by addition of

chemicals called coagents

e.g., methacrylates

This results in a higher state of cure with

improvements in properties such as

compression set


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Electron beam curing

not widely used throughout the industry

process has found a place in partially cross-

linking components of tires

radiation dosage:

about four Megarads for partial cross-linking

much higher doses for fully vulcanizing even thin

rubber sections


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Antioxidants, age resistors

and anti-degradants

Many vulcanizates become brittle when they

aged

Aging can be caused by the presence of

oxygen, accelerated by heat

Antioxidants are designed to slow down this

process and can act as free radical

scavengers


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and anti-degradants

many antioxidants are available

Selection criteria:

light colored compounds

where the product comes into contact with a

surface that can not tolerate a stain

volatility


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and anti-degradants

Antiozonants,such as p-phenylene diamines,

provide protection against ozone, and are

often added to a compound

This chemical group also has very good

antioxidant activity

Antidegradant

An antidegradant is a compounding material usedto retard deterioration caused by oxidation,

ozone, light or combinations of these

The Basic Rubber Compound

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