Peroxide Crosslinking: Detailed Method, Formulation, and Step-by-Step Process

Peroxide crosslinking is a widely used method to enhance the properties of polymers, such as improving their mechanical strength, chemical resistance, and thermal stability. Here's a detailed explanation of the method, formulation, and step-by-step process to achieve crosslinking.

1. Principle

Peroxide crosslinking involves the use of organic peroxides as crosslinking agents. These peroxides decompose upon heating to form free radicals, which then react with the polymer chains to form covalent crosslinks.

2. Formulation

A typical formulation for peroxide crosslinking might include:

Polymer: The base polymer to be crosslinked (e.g., ethylene-propylene rubber (EPR), polyethylene (PE), etc.).

Peroxide: The crosslinking agent (e.g., dicumyl peroxide, di-tert-butyl peroxide).

Coagents: Optional additives that enhance crosslinking efficiency (e.g., triallyl isocyanurate (TAIC), triallyl cyanurate (TAC)).

Fillers: Inorganic fillers such as calcium carbonate, silica, or carbon black to improve mechanical properties.

Plasticizers: Optional additives to improve flexibility (e.g., dioctyl phthalate (DOP)).

Stabilizers: Additives to protect the polymer from degradation (e.g., antioxidants, UV stabilizers).

3. Mechanism

The crosslinking mechanism involves three main steps:

Primary Radical Formation: The peroxide decomposes thermally to form free radicals.

Hydrogen Abstraction: The free radicals abstract hydrogen atoms from the polymer chains, creating new radicals on the polymer.

Radical Recombination: The newly formed polymer radicals recombine to form covalent crosslinks.

4. Step-by-Step Crosslinking Process

Step 1: Selection of Peroxide

Choose the appropriate organic peroxide based on the polymer type and desired properties. Common choices include dicumyl peroxide, di-tert-butyl peroxide, and benzoyl peroxide.

Step 2: Mixing

Mix the peroxide with the polymer in a controlled environment to ensure uniform distribution. This can be done using an internal mixer or a two-roll mill.

Add any coagents, fillers, plasticizers, or stabilizers as required by the formulation.

Step 3: Preforming

Shape the polymer-peroxide mixture into the desired form using extrusion, injection molding, or compression molding.

Step 4: Heating

Heat the formed polymer mixture to a specific temperature (usually between 150°C and 200°C) to initiate peroxide decomposition.

The heating process can be carried out in a hot press, oven, or vulcanization tunnel.

Step 5: Crosslinking Reaction

As the peroxide decomposes, free radicals are generated, which react with the polymer chains to form crosslinks.

The crosslinking reaction typically takes place over a period of minutes to hours, depending on the temperature and peroxide type.

Step 6: Cooling

After the crosslinking reaction is complete, cool the material to stabilize the crosslinked network.

Cooling can be done using air cooling, water quenching, or controlled temperature chambers.

Step 7: Post-Curing (Optional)

For some applications, a post-curing step may be needed to achieve optimal properties. This involves additional heating at a lower temperature for a specified period.

Step 8: Quality Control

Perform tests to ensure the crosslinked polymer meets the desired mechanical, thermal, and chemical properties.

Common tests include tensile strength, elongation at break, hardness, and thermal stability.

Advantages of Peroxide Crosslinking

Improved Mechanical Properties: Higher tensile strength, improved elasticity, and better resistance to deformation.
Enhanced Chemical Resistance: More resistant to chemicals, oils, and environmental stress cracking.
Thermal Stability: Higher thermal stability and can withstand higher temperatures without degradation.
Low Compression Set: Crosslinked elastomers have a lower tendency to deform permanently under load.

Applications

Cable Insulation: Crosslinked polyethylene (XLPE) is used for high-voltage power cables.
Automotive Parts: Crosslinked rubber is used in hoses, belts, and seals.
Footwear: Crosslinked EVA is used in shoe soles for enhanced durability.
Medical Devices: Crosslinked silicone rubber is used in medical tubing and implants.

Peroxide crosslinking is a versatile and effective method to enhance the properties of various polymers, making it suitable for a wide range of industrial applications.

Temperature (°C)    Pressure (bar)

100    1.013
101    1.051
102    1.090
103    1.131
104    1.173
105    1.216
106    1.261
107    1.307
108    1.355
109    1.404
110    1.455
111    1.508
112    1.562
113    1.618
114    1.675
115    1.734
116    1.794
117    1.857
118    1.921
119    1.986
120    2.053
121    2.122
122    2.192
123    2.265
124    2.339
125    2.414
126    2.491
127    2.570
128    2.651
129    2.733
130    2.817
131    2.902
132    2.990
133    3.079
134    3.170
135    3.262
136    3.356
137    3.452
138    3.550
139    3.649
140    3.750
141    3.853
142    3.957
143    4.063
144    4.171
145    4.281
146    4.392
147    4.505
148    4.620
149    4.737
150    4.855
151    4.975
152    5.097
153    5.221
154    5.346
155    5.473
156    5.602
157    5.733
158    5.865
159    6.000
160    6.136
161    6.273
162    6.413
163    6.554
164    6.697
165    6.842
166    6.988
167    7.137
168    7.287
169    7.438
170    7.592
171    7.747
172    7.904
173    8.063
174    8.223
175    8.385
176    8.549
177    8.715
178    8.882
179    9.051
180    9.222
181    9.394
182    9.568
183    9.744
184    9.921
185    10.100
186    10.281
187    10.464
188    10.648
189    10.834
190    11.021
191    11.210
192    11.401
193    11.593
194    11.787
195    11.983
196    12.180
197    12.378
198    12.579
199    12.781
200    12.985
201    13.191
202    13.398
203    13.606
204    13.817
205    14.029
206    14.242
207    14.458
208    14.675
209    14.894
210    15.114
211    15.336
212    15.560
213    15.785
214    16.012
215    16.240
216    16.470
217    16.702
218    16.936
219    17.171
220    17.408
221    17.646
222    17.886
223    18.128
224    18.371
225    18.616
226    18.862
227    19.110
228    19.359
229    19.610
230    19.862
231    20.116
232    20.372
233    20.629
234    20.888
235    21.148
236    21.409
237    21.673
238    21.938
239    22.204
240    22.472
241    22.742
242    23.013
243    23.285
244    23.559
245    23.835
246    24.112
247    24.391
248    24.671
249    24.953
250    25.237