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Mate­r­i­al selec­tion in rub­ber production — 
Which com­po­nents are decisive? 

01.09.2021   |   Dr. Robert Zahn

Verschiedene Polymere

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Rub­ber is a ver­sa­tile mate­r­i­al that is suit­able for a wide range of indus­tri­al appli­ca­tions due to its chem­i­cal and phys­i­cal prop­er­ties. This ver­sa­til­i­ty results, among oth­er things, from the fact that a rub­ber com­pound is a com­plex mix­ture of sev­er­al com­po­nents, each of which can sig­nif­i­cant­ly influ­ence the prop­er­ties of the vul­can­izate. Find­ing the one com­pound that meets every vul­can­izate require­ment is often a chal­lenge, because opti­miz­ing one fac­tor can neg­a­tive­ly affect the oth­ers. There­fore, man­u­fac­tur­ing com­pa­nies should not under­es­ti­mate the com­plex­i­ty of mate­r­i­al selec­tion in the rub­ber sector. 

Rub­ber is not only made of rubber

A rub­ber com­pound con­sists of a vari­ety of raw mate­ri­als and addi­tives that have a sig­nif­i­cant influ­ence on both the pro­cess­ing and the prop­er­ties of the final prod­uct. In addi­tion to the rub­ber, these include fillers, plas­ti­ciz­ers, the cross-link­ing sys­tem, pro­cess­ing aids and var­i­ous addi­tives (see Table 1). 
Com­po­nent
Effect on the properties
Rub­ber
Basic prop­er­ties (tem­per­a­ture behav­ior, chem­i­cal resis­tance, elasticity, …)
Filler
Hard­ness, strength, abra­sion, elec­tri­cal conductivity
Plas­ti­ciz­er
Hard­ness, work­a­bil­i­ty, elon­ga­tion at break
Cross-link­ing system
Vul­can­iza­tion speed, dynam­ic and sta­t­ic behavior
Pro­cess­ing aid
Flow behav­ior, demould­abil­i­ty, pro­cess­ing behavior
Addi­tive
Col­or, extru­date sta­bil­i­ty, age­ing behav­ior, elec­tri­cal conductivity, …

Table 1: The most impor­tant com­po­nents of a rub­ber com­pound and their effect on the prop­er­ties of the vulcanizate.

The basic com­po­nent of every rub­ber com­pound is the poly­mer, the rub­ber, which deter­mines basic prop­er­ties such as tem­per­a­ture behav­ior, chem­i­cal resis­tance or elas­tic­i­ty. 

When putting togeth­er a rub­ber com­pound, the man­u­fac­tur­er has a vari­ety of syn­thet­ic rub­bers at his dis­pos­al in addi­tion to nat­ur­al rub­ber, each of which has spe­cif­ic prop­er­ties. Some rub­bers, for exam­ple, offer excel­lent oil resis­tance, oth­ers impress with out­stand­ing mechan­i­cal prop­er­ties, and still oth­ers with very high age­ing resis­tance. For most appli­ca­tions, there is a suit­able rub­ber that offers the desired prop­er­ties. 

Besides rub­ber, fillers (e.g. car­bon black, sili­cic acid or chalk) make up the largest pro­por­tion of a rub­ber com­pound. Among oth­er things, they influ­ence the hard­ness, strength, abra­sion and elec­tri­cal con­duc­tiv­i­ty of the vul­can­ized rub­ber. Plas­ti­ciz­ers also influ­ence the hard­ness and work­a­bil­i­ty of the com­pound, but also the stretch­a­bil­i­ty. Although the cross-link­ing sys­tem makes up only a small part of the rub­ber com­pound, it has a great influ­ence on its prop­er­ties. It pri­mar­i­ly con­trols the speed of vul­can­iza­tion, but also influ­ences oth­er char­ac­ter­is­tics such as tear resis­tance. Pro­cess­ing aids influ­ence the han­dling of the rub­ber com­pound dur­ing the pro­duc­tion process. These include, for exam­ple, tack­i­ness and flow behav­ior, i.e. how eas­i­ly the com­pound can be pressed into a mold. In addi­tion, there are a large num­ber of oth­er addi­tives that influ­ence, among oth­er things, the col­or, the extru­date sta­bil­i­ty, the age­ing behav­ior and the elec­tri­cal con­duc­tiv­i­ty of the rub­ber compound.

The selec­tion of the net­work­ing system

The clas­sic crosslink­ing com­po­nent for vul­can­iza­tion is sul­fur. How­ev­er, pure sul­fur is not very effi­cient for crosslink­ing. There­fore, in prac­tice, accel­er­a­tors are added to it to acti­vate it and ensure that it is sen­si­bly incor­po­rat­ed into the poly­mer chain. Depend­ing on the ratio between sul­fur and accel­er­a­tor, a dis­tinc­tion is made between three types of vulcanization:

All three approach­es dif­fer in terms of cost, crosslink­ing speed and phys­i­cal prop­er­ties of the vul­can­izate (see Fig­ure 1). 
Fig­ure 1: Com­par­i­son of con­ven­tion­al, semi-effi­cient and effi­cient vulcanization.

Con­ven­tion­al vulcanization 

Con­ven­tion­al vul­can­iza­tion uses a lot of sul­fur and lit­tle accel­er­a­tor. This method is char­ac­ter­ized by good dynam­ic prop­er­ties and high strength. It is also some­what less expen­sive than the oth­er two types of vul­can­iza­tion. A dis­ad­van­tage is a cer­tain ten­den­cy to rever­sion, i.e. if the vul­can­iza­tion process is too long, the prop­er­ties dete­ri­o­rate, which in turn affects the pro­duc­tion process. 

Effi­cient vulcanization 

In con­trast to con­ven­tion­al vul­can­iza­tion, effi­cient vul­can­iza­tion uses more accel­er­a­tor and less sul­fur. This increas­es the crosslink­ing speed, while the ten­den­cy to rever­sion decreas­es. This type of vul­can­iza­tion is char­ac­ter­ized by good com­pres­sion set and high heat sta­bil­i­ty. How­ev­er, the cost is some­what high­er, as accel­er­a­tors are more expen­sive com­pared to sulfur. 

Semi-effi­cient vulcanization 

In prac­tice, a hybrid approach is often used, i.e. a bal­anced ratio of sul­fur and accel­er­a­tors, as this approach has no sig­nif­i­cant weak­ness­es. This is a clear advan­tage in prac­tice, since one usu­al­ly has to con­sid­er a large num­ber of dif­fer­ent require­ments here. 

Case Study: Spring ele­ments for agri­cul­tur­al technology

In our Case Study, you can learn what intel­li­gent mate­r­i­al selec­tion can achieve in practice.

The den­si­ty of networking

In addi­tion to the ratio between the two com­po­nents, the pure amount of crosslink­ing chem­i­cals is also rel­e­vant, because the prop­er­ties of the vul­can­izate also change with the num­ber of crosslink­ing points per unit vol­ume (see Fig. 2). If the crosslink den­si­ty increas­es, both hard­ness and elas­tic­i­ty increase, while prop­er­ties such as com­pres­sion set decrease. Oth­er aspects such as ten­sile strength, on the oth­er hand, do not devel­op lin­ear­ly. They ini­tial­ly increase, then reach a max­i­mum and decrease again as the crosslink den­si­ty con­tin­ues to increase. For this rea­son, it is impor­tant to adapt the amount of crosslink­ing sys­tem exact­ly to the gen­er­al con­di­tions. In this con­text, “a lot helps a lot” does not apply and may even prove harmful. 
Fig­ure 2: Effect of crosslink­ing den­si­ty on var­i­ous prop­er­ties of the vulcanizate. 

The filler system

Anoth­er impor­tant com­po­nent to con­sid­er in the con­text of mate­r­i­al selec­tion is the filler sys­tem. Here we dis­tin­guish between active (or also rein­forc­ing) and inac­tive fillers:

An overview of the influ­ence of active and inac­tive fillers on vis­cos­i­ty, hard­ness, tear strength and abra­sion of a vul­can­izate can be tak­en from Fig­ure 3. 
Fig­ure 3: Overview of the effects of rein­forc­ing and inac­tive fillers on the prop­er­ties of the vulcanizate. 

In sum­ma­ry: Intel­li­gent mate­r­i­al selec­tion is a must

In addi­tion to the base mate­ri­als rub­ber and sul­fur, there are a large num­ber of oth­er com­po­nents that go into the pro­duc­tion of a vul­can­izate. Each of these mate­ri­als has a dif­fer­ent effect on the prop­er­ties of inter­me­di­ate and end prod­ucts. Through inter­ac­tion between the com­po­nents, they col­lec­tive­ly form a web of influ­enc­ing fac­tors, the com­plex­i­ty of which man­u­fac­tur­ing com­pa­nies should not underestimate.

Find­ing a mix that meets all require­ments is often a chal­lenge, because opti­miz­ing one fac­tor can have a neg­a­tive impact in oth­er areas. The prin­ci­ple of “a lot helps a lot” there­fore does not apply in rub­ber pro­duc­tion. Instead, com­pa­nies should make sure to focus on the prop­er­ties of the fin­ished prod­uct as ear­ly as the mate­r­i­al selec­tion stage. Intel­li­gent deci­sions at this ear­ly stage pay off in the long run.

Whitepa­per:
Secure the Start of Production 

Learn which fac­tors influ­ence your SOP! 

Author: Dr. Robert Zahn

Our com­pound­ing expert Dr. Robert Zahn com­plet­ed his doc­tor­ate at Leib­niz Uni­ver­si­ty in Hanover, Ger­many, in coop­er­a­tion with the Ger­man Insti­tute for Rub­ber Tech­nol­o­gy e.V. (DIK). His focus was on elas­tomer chem­istry and rub­ber tech­nol­o­gy. He has been work­ing for Jäger at the Hanover loca­tion since 2017 in prod­uct and process devel­op­ment for the area of mate­r­i­al development. 

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