Antioxidants in plastic industry

Antioxidants in plastic industry

Introduction:
Objects are made of polymer have possibility of oxidation degradation and their service life will be short. Oxidation degradation can lead to:
-Discoloration
-Loss of sharpness
-Loss of tensile strength-Strength loss beats-Loss of polymer properties
(AO): abbreviation of antioxidant
In general, polymeric materials are sensitive to oxygen and oxidize that include:
Synthetic polymers
Natural polymers
Differentiations of polymers resistance against oxidation are very different:
High resistance: poly Astray and poly methyl methacrylate
Low resistance: Polymeric unsaturated elastomers
Methods to prevent oxidation of the polymer:
Modify the structure of the polymer: 
Coating end groups (mostly for poly acetyl)
Sustainable physics simulation by arrow polymer (B
Adding AO: the most common method of stabilization of polymers
Definition or AO:
Material which have special chemical structure that show their effects in low concentration and because  compared to polymers
have more speed in react with free radicals,  prevent  from oxidation polymer.
ROO● + AH → ROOH +A ● (K17)
ROO ● + RH → ROOH + R ● (K3)
AH: antioxidant
RH: polymer
AO must have some properties:
- In powder form and are easily melt (appropriate distribution)
-Don't cause discoloring
Has stopped the process of destruction by copper, magnesium and other metals-
Not toxic-
Not volatile-
Oxidation polymers-
-The process of oxidation damage caused by free radicals reacts with polymers, which consists of 4 stages
Start stage
Publishing stage-
Self-
catalyst stage-
Final stage
Start stage:
Free radicals which start the reaction occur as different types (light, temperature...) in the polymer because polymers have minor amounts of peroxide is as follows: The reaction for substance with the general formula of RH
RH→ HO●, RO●, ROO●, R●
Ultraviolet light is absorbed by hydro peroxides is the main source of the production of free radicals.
Publishing stage:
Before proxy or alkyl radicals become to non-radical products can be repeated and caused would be soon starting stage:
R● + O2 → ROO●
ROO● + RH → ROOH + R●
Self-catalyst stage:
Oxidative degradation of the polymer usually is low at the beginning of life, but over time it increases due to the formation of radicals which arise through decomposition of hydro peroxides:
ROOH   → RO ●   + HO ●
2ROOH→ RO ● + ROO ● + H2O
ROOR→ 2RO●
Final stage:
When Proxy and alkyl radicals became to non-radical products it causes to end publishing stage reactions:
ROO● + R● → POOR
2R ● → R-R
Type of antioxidants:
1- The first type of antioxidants.
2 - The second type of antioxidants.
The first kind Antioxidants neutralize them by giving hydrogen usually from OH or NH to free radical
Phenol inhibitor
Aromatic amines:
There is the highest diversity among the substances AO in inhibiting phenols family. One of the most common of these compounds, is hydroxyl toluene v (BHT) that use in types of polymers including PS, PP, PE
AR-OH + ROO●→AR-O● + ROOH
Aromatic amines:
Howevr, aromatic amines are stronger than phenols, but they were not used in thermoplastic film because of blemish negative properties .The major use of aromatic amines is in curable elastomers.
-NH + ROO● → -NO● + ROOH
Second type of antioxidants
The second type of antioxidants protects polymers with degradation of peroxide radicals.
1-Thioesters are oxidized to sulfones:
(ROCOCH2CH2)2S +ROOH → -NO● + ROOH
2- Phosphite is oxidized to phosphate:
PR'3 +ROOH → POR'3 +ROH
The effect of antioxidants in polymers:
To study the effect of antioxidant is done OIT test. In this test, the oxidation time is measured in minutes.
As in the chart below is clear, adding Irganox×1010  antioxidant in 0.5  levels increased oxidation time to 120 minutes.



Determine the oxidation temperature
To determine the oxidation point of polymers, OIT test is done. As shown in the following graph, sample has an exothermic eaction at 230/9 ° C, which represents its oxidation point.



Synergism effects:
To achieve a better result usually is used a mixture of two or more antioxidants.
In fact, using of a first type antioxidant with a second type anti-oxidant is resulted synergism effects and is caused further protecting 
gainst single AO.

An example of polymer tube degradation by oxidation:



Summary:
Polymers have the possibility of oxidative degradation and their service life is short. The process of oxidative degradation caused by free radicals reacts with the polymer which is divided into four stages:
1- Start stage   2. Diffusion stage   3-catalyst stage   4. Final stage
There are several ways to prevent oxidation polymers which the most common is to add antioxidants AO
Antioxidants are of two types: type I and type II 
The first type of antioxidant: by giving hydrogen usually from OH or NH to free radical,
that neutralizes them, and includes phenols deterrent and aromatic amines.
The second type of antioxidants: protects the polymers by destruction of peroxide radicals and include1 Phosphate and 2-thioester.
To achieve a better result, usually use a mixture of two or more antioxidants, which is called synergism effects.
To test the effect of antioxidants in polymers is performed test of OIT and to determine temperature or oxidation polymer is performed test of DSC.

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