18
2 Filler and Rubber Reinforcement
Here, ‘India rubber’ was the name for NR then. What he pointed out as ‘the further
questions’ on vulcanization remaining at the time, are still under active investigation
by one of the present authors [32–36].
More important for rubber reinforcement is Weber’s critical comment on rubber
ingredients also written in the book [31]: He described enormous amount of painstaking works on additives onto NR, and tabulated chemical and physical test results on
a lot of inorganic and organic compounds in pp. 164–174, and summarized them in
seven tendencies at p. 175. However, he also wrote there,
I fear that these tables do not give us any very reliable indication of the influence of the
inorganic and organic admixtures experimented with upon the manufactured India rubber.
He therefore proposed that these additives were better to be rationally grouped into
three; Fillers, sulfur compounds, and pigments. His proposal was in accordance with
their functions, i.e., the three are for reinforcement, vulcanization, and coloring,
respectively.
Though the progress was agonizingly slow, his proposition had been gradually
accepted, and the title of Chap. 5 of the book [2] published in 1952 is as follows:
Compounding Ingredients:
(i) Fillers, reinforcing agents, colors, softeners, plasticizers and extenders.
(ii) Accelerators of vulcanization, antioxidants, and vulcanizing agents.
where fillers, reinforcing agents, extenders are differentiated in (i), and vulcanization
reagents are separately described in (ii).
By the end of 1960s, more or less standard formulations and processing procedures
were established. In other words, rubber engineers had a clear sense of purpose in the
selection of additives and were able to figure out an appropriate procedure in accord
with the selected recipe [2, 6–8, 10, 12, 37–39]. Note that the distinction between
filler and pigments was not well recognized until 1950s.
At present, the reagents of sulfur vulcanization include sulfur, accelerators, activators (stearic acid and zinc oxide are most common now), scorch (or pre-vulcanization)
inhibitors, and strangely even retarders (e.g., phthalic anhydride) often together with
accelerator! Weather-proof reagents include antioxidants, antiozonants, ultraviolet
absorbers, and wax or paraffin. For facilitating the mechanical mixing of rubber,
various additives are available, for example, plasticizers, softeners, and lubricants.
Often they are called processing aids in general. Sometimes, peptizers are in use in
order to plasticize or soften rubber just before compounding with any other materials. The peptizer is assumed to prevent rubber from the recombination reactions
between the mechanochemically decomposed fragments during mechanical mixing.
After the shaping of rubber compounds in accordance with the product, the final
shape forming is done at the vulcanization step, using a metallic mold under heating,
and the use of an appropriate reagent to reduce the contamination of mold surface
and/or mold release agents (for the easy release of rubber products from the mold)
is quite common in order to protect both the mold and the product.
Further, when not vulcanization (which is the word for cross-linking using sulfur) but the cross-linking reaction by use of peroxide, i.e., free radical mechanism is
2 Filler and Rubber Reinforcement
Here, ‘India rubber’ was the name for NR then. What he pointed out as ‘the further
questions’ on vulcanization remaining at the time, are still under active investigation
by one of the present authors [32–36].
More important for rubber reinforcement is Weber’s critical comment on rubber
ingredients also written in the book [31]: He described enormous amount of painstaking works on additives onto NR, and tabulated chemical and physical test results on
a lot of inorganic and organic compounds in pp. 164–174, and summarized them in
seven tendencies at p. 175. However, he also wrote there,
I fear that these tables do not give us any very reliable indication of the influence of the
inorganic and organic admixtures experimented with upon the manufactured India rubber.
He therefore proposed that these additives were better to be rationally grouped into
three; Fillers, sulfur compounds, and pigments. His proposal was in accordance with
their functions, i.e., the three are for reinforcement, vulcanization, and coloring,
respectively.
Though the progress was agonizingly slow, his proposition had been gradually
accepted, and the title of Chap. 5 of the book [2] published in 1952 is as follows:
Compounding Ingredients:
(i) Fillers, reinforcing agents, colors, softeners, plasticizers and extenders.
(ii) Accelerators of vulcanization, antioxidants, and vulcanizing agents.
where fillers, reinforcing agents, extenders are differentiated in (i), and vulcanization
reagents are separately described in (ii).
By the end of 1960s, more or less standard formulations and processing procedures
were established. In other words, rubber engineers had a clear sense of purpose in the
selection of additives and were able to figure out an appropriate procedure in accord
with the selected recipe [2, 6–8, 10, 12, 37–39]. Note that the distinction between
filler and pigments was not well recognized until 1950s.
At present, the reagents of sulfur vulcanization include sulfur, accelerators, activators (stearic acid and zinc oxide are most common now), scorch (or pre-vulcanization)
inhibitors, and strangely even retarders (e.g., phthalic anhydride) often together with
accelerator! Weather-proof reagents include antioxidants, antiozonants, ultraviolet
absorbers, and wax or paraffin. For facilitating the mechanical mixing of rubber,
various additives are available, for example, plasticizers, softeners, and lubricants.
Often they are called processing aids in general. Sometimes, peptizers are in use in
order to plasticize or soften rubber just before compounding with any other materials. The peptizer is assumed to prevent rubber from the recombination reactions
between the mechanochemically decomposed fragments during mechanical mixing.
After the shaping of rubber compounds in accordance with the product, the final
shape forming is done at the vulcanization step, using a metallic mold under heating,
and the use of an appropriate reagent to reduce the contamination of mold surface
and/or mold release agents (for the easy release of rubber products from the mold)
is quite common in order to protect both the mold and the product.
Further, when not vulcanization (which is the word for cross-linking using sulfur) but the cross-linking reaction by use of peroxide, i.e., free radical mechanism is