Processing additives - A to Z.

Overview

Processing additives have been used throughout the years to improve the versatility of elastomers. In fact, table 1 lists all of the positive claims for processing additives. They include benefits in mixing, including mills and internal mixers, calendering, extrusion, building operations and molding. This series of articles will review the history, the general classes of additives, the chemistry and briefly describe the various modes of action. Specific examples will be presented showing how these additives can be of benefit to compounders.

Table 1-claims for processing additives

                                 Output       Quality

Improved filler dispersion                     X
Reduced mixing time                X
Reduced mixing energy              X
Better mill handling               X           X
Faster extrusion                   X
Lower heat build-up                X           X
Reduced die swell                  X           X
Better calendering                 X           X
Shorter injection times            X           X
Improved mold release              X
Easier fabrication                 X
Improved product appearance                    X

History

What is a processing additive?

A processing additive is a material which, when added to a rubber compound at relatively low loading, will improve processability without adversely affecting physical properties.

History of processing additives

It is difficult to determine who knowingly first used processing additives to modify a natural or synthetic polymer. In 1865 Alexander Parks obtained a patent for the use of various materials, including fatty glycerides, oils, gum and tars in lubricating nitro-cellulose: and John and Isaiah Hyatt obtained a patent on the use of camphor in nitro-cellulose in 1870. On May 11, 1858. H.L. Hall in Massachusetts was assigned U.S. Patent 220242 to Beverly Rubber Company for the use of asphalt, coal tar, resin and pitch to convert hard vulcanized rubber into a soft material which could be formed into useful articles (ref. 1).

Categories/types

Classification of processing additives

In one of the most recent editions of the Blue Book: materials, compounding ingredients, machinery and services for the rubber industry, under the category of processing aids, are the following sub-categories: homogenizing agents, peptizers, processing aids, tackifiers, plasticizers and softeners In recent years, there has been a proliferation of materials classified as processing aids or processing agents (table 2). The compounder, in designing a compound, carefully selects the elastomer system, reinforcing agent, protective agents, accelerators and vulcanizing agents that are needed to impart to the compound's specific performance properties. There are numerous polymers vulcanizing agents, reinforcing agents and protective agents from which the compounder can select. Each is chemically designed to perform a specific function in the compound. Selection of the best processing aid for a particular application can be a problem for rubber compounders. First we will review the different types of additives available to compounders today.

Table 2 - number of processing aids - 1980-1997

                               1980     1984     1997

Homogenizing agents               4       8       12
Peptizers                        14      21       35
Processing aids                  82     167      263
Tackifiers                      152     167      123
Plasticizers and softerners     400     450      532
Total                           652     813      965

Plasticizers and softeners

"Plasticizers and softeners are relatively non-volatile, organic chemicals that, when added to elastomers or resins, will improve workability during fabrication or processing; extend or modify inherent properties, especially increased flexibility and distensibility at low temperatures; and they are products used: to develop new and improved properties," (ref. 1).

Plasticizers can be classified under the following categories and sub-categories:

* Hydrocarbon oils are petroleum-based oils. In rubber compounding they are the largest used of all the other categories listed by a ratio of 4:1. These universal oils are used to improve now and processing characteristics while reducing cost of the final compound of petroleum-based oils include naphthenic, paraffinic and aromatic oils. The ratio of use by each is given in table 3: Within this category we find two other classes of materials.

Table 3-ratio of uses of oil

As extenders                        As process oils

Aromatic           75-80%           Aromatic            30%
Naphthenics        15-20%           Naphthenics         60%
Paraffinics            5%           Paraffinics         10%

1. Coal tar by-products were first used by rubber compounders to soften rubber, but they were soon replaced by oils from petroleum. Coal tar oils are produced from the coking of coal and are very complex mixtures of compounds. They are viscous, water-white to light yellow liquids with solvent and plasticizing properties. Included are low molecular weight coumarone-indene compounds.

2. Pine tar and resins are added to impart tack, soften the compound, improve flow and in some cases, improve wetting out. Their production today has been reduced considerably, so) much that a shortage of native pine tar and rosin exists in the marketplace.

* Organic esters are unique in the sense that the ester group (-COO-) contributes an unusual property to an organic molecule, allowing it to be compatible in many polymers, be less viscous and contribute low temperature, softening and solvent power to an otherwise passive hydrocarbon molecule. Organic esters are primarily used in polar elastomers, such as polychloroprene and nitrile rubber, to improve low and high temperature performance or impart particular oil or a solvent resistance to a compound. Monomeric ester is made from compounds containing at least one mono-functional group. They may be on the organic acid, the organic alcohol or both. Polymeric esters (polyesters) are made from carboxylic acid and hydroxylic compounds each having two or more groups so that they may react forming a linear chain. Polyester typically has high viscosity and molecular weights up to 8,000. Other important analyses which can be used to characterize the ester are:

1. Saponification number can be a tip-off of its type: mono-functional, all-functional or polymer. It is also a good indication of its polarity or solvency. The higher the saponification number the more polar the plasticizer.

2. Iodine number will determine if it contains an unsaturated fatty acid and may be a factor in its color stability.

3. Specific gravity may indicate degree of branching and, of course, is important from a consideration of pound-volume cost. Usually the low specific gravity will give more manufactured parts per unit weight and small differences may result in significant economic savings. Polarity generally causes increases in specific gravity because polar atoms are heavier than carbon atoms.

4. Freezing point gives an idea of its temperature lowering effect on the polymer.

5. Viscosity gives an idea of its efficiency as a plasticizer.

6. Flash point gives an idea of its permanence.

7. Acid value may be a characteristic important to its stability and curing (ref. 1).

Table 4 shows the principal compounds used in the manufacturing of esters.

Table 4-principal compounds used in manufacture of esters

                            Organic acids

Monobasic acids           Dibasic acids          Tribasic acids
Oleic                     Phthalic anhydride     Trimellitic
                                                 anhydride

Stearic                   Adipic acid                Citric acid
Benzoic                   Sebacic acid
Pelargonic                Azelaic acid
                          Glutaric acid

                            Organic alcohols

Monohydric                Dihydric               Trihydric
2-Ethylhexanol            Propylene glycol       Glycerol
Isodecanol                1,3 butandiol          Trimethylolpropane
6-10 linear alcohols      1,4 butandiol
Benzyl alcohol
Isononyl alcohol

* Polymerics include a number of lower molecular weight resins, plastics and elastomers which are useful as plasticizers. In some cases, a lower molecular weight version of the polymer to be plasticized will be used as the plasticizer. This makes good sense since compatibility problems would not be expected. Some of these polymers available today are: low molecular weight nitrile rubber (Zeon 1312); high molecular weight polyisobutylenes (Vistanex); EVA terpolymers; high and low crystalline types-polyoctenamer (Vestenamer).

* Polar-specialty types include three categories:

1. Phosphates are formed by the reaction of alcohol with phosphorous oxide, or more desirably with acid chlorides. These plasticizers have a two-fold function. They are good plasticizers and they contribute excellent fire retardant properties. Examples include phenyl and cresyl phosphates.

2. Another type exists called epoxy compounds. Examples of these include epoxidized soybean and linseed.

3. Also under the polar-specialty group is a group made up of polyethylene glycol ester and ether compounds. Examples include Di-(butoxyethoxy) phthalate, Di-(butoxyethoxy) adipate, and Di-(butoxyethoxy) sebacate.

Chemical types

As earlier stated plasticizers and their derivatives can truly be considered processing additives, however, most are added as a basic compounding ingredient to achieve a property or cost that the compounder desires. In the mid-1900s, a new class of materials was designed specifically to enhance processing characteristics. These chemicals are called process and dispersing agents.

References

(1.) Dr. Vince Kuceski, "Plasticizers" (Chicago, IL).

(2.) AlliedSignal Technical Reprint CTG027 by Dr. Eric H. Erenrich and Marco A. Garcia Lopez