Processing additives - A to Z.

Amides

Amides are any member of either of two classes of nitrogen-containing compounds related to ammonia and amine. Simple amides ordinarily are prepared by reaction of acids or acid halides with ammonia or amines. They can also be produced by the reaction of water with nitriles. The characteristic reaction of covalent amides is hydrolysis (a chemical reaction with water), by which they are converted to acids and amines. Amides also can be dehydrated to nitriles. Amides are not readily oxidized nor reduced, although hydrogenation (addition of hydrogen at high temperatures and pressures) in the presence of a catalyst will convert most amides of carboxylic acids to amines. The powerful reducing agent lithium aluminum hydride transforms amides into amines. Two very important uses of amides and amines in plastic, and now rubber compounding, are as an antistat, slip and antiblock.

Static electricity is a surface phenomenon. Dissipation of a static charge is normally achieved by decreasing the surface resistivity of the polymer. This is done by using surface active additives, such as antistats. Due to the hygroscopic nature of these additives, a conductive layer is formed on the polymer surface with a thin film of water from the atmosphere. This process allows the charge to dissipate rapidly. Thus, humidity plays a vital role in this mechanism. With increasing humidity, the surface conductivity of the polymer is increased, resulting in a rapid flow of charges and better antistatic properties (ref. 3). Based on their mode of application, there are two types of surface active antistatic additives, namely, internal and external.

Internal antistatic additives are compounded into the resin matrix during processing. These surface active additives have the ability to replenish the polymer's antistatic protection which can be worn off during handling (refs. 4 and 5). The mode of action of internal antistats depends on blooming. Blooming is defined as a process whereby a partially compatible component, in this case the internal antistat, migrates to the surface of the polymer. The internal antistats, therefore, offer long-term protection against accumulation of static electric charges (refs. 6 and 7). By far the largest class of internal antistatic additives is ethoxylated fatty amine (ref. 8).

Amides are based on fatty acids derived from naturally occurring feedstocks, such as tallow and rapeseed. Primary amides such as stearamide, oleamide and erucamide are internal lubricants having slip and antiblocking properties. They migrate to the surface of a polymer due to their limited compatibility and form an ultra-thin layer that reduces the friction (slip effect) and adhesion forces (antiblocking effect). They offer a solution to processing problems caused by the inherent friction and tackiness of polymers. These problems include slip-stick effect, sticking of film layers to each other (blocking), mold contamination and excessive equipment wear. These problems often result in lower production output and reduced performance during service life. Other categories include:

* Amorphous polypropylene;

* castor oil and its derivatives;

* fluorinated products -- fluorochemical and modified PTFE;

* organosilicone -- blends and combinations of fatty acids and silicone;

* silicones;

* sodium alkylsulphates;

* stearates;

* vulcanized vegetable oils.

Natural synthetic resins

Natural synthetic resins are products originating from thermal polymerization of side products or residues of petroleum or coal industries. These include aliphatic/naphthenic/ aromatic resin blends and combinations; polar surfactants, comprising of mixture of barium salts of tall oil bound to inorganic fillers; polar surfactants on high activity silica; and processing lubricants with surfactant activity.

Homogenizing agents

Homogenizing agents are included in a large class of products listed under this classification. Homogenizing agents are low molecular weight polymeric resin blends. They assist the widely dissimilar ingredients used in a rubber compound to coalesce and mix into a homogenous uniform processable mass. Each type of elastomer has a plasticizer or plasticizers which are effective in lowering the viscosity or softening the elastomer. In figure 6, some common elastomers have been listed, along with the type of plasticizer that is most effective at softening it. Note that the solubility parameters and polarities of the plasticizers closely match those of the elastomer in which it is most effective. The further apart any two elastomers are in solubility parameter or viscosity, the more difficult they are to get to produce a homogenous mix. Blends of plasticizers, each compatible with different elastomers being blended, are effective at improving blend homogeneity. Plasticizer blends for homogenizing have the disadvantage of being prone to migration and bloom. To overcome the migration problem and still obtain the homogenizing effect, the homogenizing agents contain higher molecular weight homologues of the plasticizers, i.e. resins. The homogenizing resin blend contains portions that are compatible with aliphatic, naphthenic and aromatic parts of the elastomers in a blend. They have a wetting effect, and thereby reduce the energy necessary to get dissimilar components to mix more easily. The polarity and molecular weight of homogenizing agents are shown compared to oils and resins in figures 7 and 8.

Peptizers

It has been known since the 1800s that natural rubber could be broken down by milling. It was discovered that oxygen was necessary during milling to obtain necessary breakdown. Peptizers are generally radical acceptors or oxidation catalysts, which effectively remove the free radicals formed during the milling and mixing procedures. This action thus prevents recombination, resulting in a plasticizer effect on the polymer. Thiophenols, aromatic disulfides and chelate complexes of iron, cobalt, copper and manganese catalyze the thermo-oxidative characteristics of some elastomers. Sulfonate esters are also considered mild peptizers for natural rubber. Table 8 lists peptizers available in the current market place.

Table 8 -- peptizers

Akrochem peptizer 66         "activated" dibenzamido
                             diphenyldisulfide on inert carrier
Pepton 44                    "activated" dibenzamido
                             diphenyldisulfide on inert carrier
Pepton 80                    "activated" dibenzamido
                             diphenyldisulfide on inert carrier
Renacit 11                   "activated" dibenzamido
                             diphenyldisulfide on inert carrier
Struktol A 95                "activated"
                             pentachlorothiophenol on inert carrier
Aktiplast 8                  "activated" mixture of fatty
                             acids, fatty acid soaps and fatty
                             alcohols
Aktiplast F                  "activated" combination of zinc
                             salts of higher molecular
                             weight, mainly unsaturated fatty
                             acids
Aktiplast M                  "activated" combination of zinc
                             salts of higher molecular
                             weight, mainly unsaturated fatty
                             acids
Renacit 8                    "activated" blend of unsaturated
                             fatty acids
Struktol A 80                "activated" mixture of high molecular
                             weight fatty acids
Struktol A 82                "activated" mixture of fatty acid
                             salts
Struktol A 91                "activated" mixture of fatty acid
                             zinc salts
Struktol A 86                blend of organo-metal
                             complexes, dispersing agents
Struktol A 88                blend of organo-metal
                             complexes, dispersing agents
Akrochem peptizer 9344       arylamine hydrocarbon mixture
Akrochem peptizer 9349       modified aryl sulfide
                             hydrocarbon mixture
Akrochem Proaid Pep          Zinc salts of high molecular
                             weight fatty acids
Reogen                       mineral oil/sulfonate blend
Bondogen                     mineral oil/sulfonate blend
Peptizer 7010N               mineral oil/sulfonate blend
Vanplast PL                  mineral oil/sulfonate blend
Vanplast R                   mineral oil/sulfonate blend

Modes of action (table 9)(ref. 9)

Limited compatibility (bloom-slippage) Medium to low compatibility (slippage) High compatibility

Intermacromolecular action -- reducing the entangle

ments between polymer molecules.

Intramacromolecular action -- swelling and softening

polymer molecules.

Limited compatibility

The product blooms to the surface of the rubber and acts as an external lubricant at the rubber-metal interface producing slippage in the equipment.

Medium to low compatibility

Below a certain concentration, the material acts as an internal lubricant causing slippage between the flow units of the compound. If the compatibility limit is exceeded, the product also acts as an external lubricant.

High compatibility

No blooming occurs and the product acts as a bulk viscosity modifier by either of the following mechanisms:

* Intermacromolecular action -- the reduction of the entanglements between polymer molecules. Peptizing.

* Intramacromolecular action -- the swelling of the polymer molecules and softening them. Plasticization.

Tribology interaction

Adhesive/lubricant/surfactants (ref 10).

Examples in elastomers case studies Summary (table 10)

We have reviewed a very large class of compounding ingredients called processing additives. I hope that every one of you will have a better understanding of what these products can and cannot do, and that their use is not magic but solid technology.

Table 10 -- processing additives according to their effect

Antistat Amines Antiblocking Primary amides Assembling agents Vulcanized vegetable oils Dispersant Fatty acids

Metal soaps

Waxes

Fatty alcohols Homogenizers HC resins

Fatty acid derivatives

Phenolic resins Lubricants Metal soaps

Fatty acid amides

Fatty acid esters

Low-molecular HC Mastication aids Aryldisulphides

Pentachlorothiophenol

Zinc salts unsaturated fatty acids

Sulphonic acids Reinforcing agents Polymer agents

Phenolic resins Tackifiers Phenolic resins

HC resins

References

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

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

[3.] Pfahler, G., "Plastic additives handbook," Gachter R., Muller H., Eds., Hanser Publishers, New York, 3rd Edition, 1990, Chapter 13, 749-772.

[4.] Polzhofer, K., Lehman H.D., "Angew. Makromol. Chem., " 1974, 36,145.

[5.] Rombusch, K. Maahs G., "Angew. Makromol. Chem.," 1973, 34, 50.

[6.] Sharma, A.H., "Plastics handbook," edited by Modern Plastics Magazine, McGraw-Hill, Inc., New York, 1994, ISBN 042805-0: Antistats, 94-96; also see van Drumpt, J.D., "Modern plastics encyclopedia," McGraw-Hill, New York, 1992, Mid-October 1992 issue, Chemicals and Additives, Antistats, 150-154.

[7.] Balbach, G., "Kunststoffe," 1977, 67, 154.

[8.] Reck, R.A., "Encyclopedia of polymer science and engineering, "John Wiley & Sons, New York, 1985, 2, Antistatic Additives, 99-115.

[9.] D. G. Lloyd, "Additives in rubber processing," (Monsanto Europe S.A., Brussells, Belgium, Reprinted in Rubber Developments, Vol. 43, No. 314, 1990, 26-33.

[10.] Dr. JP. Vander Kooi, "Role of additives in processing of polymers," (ACS Rubber Division, Montreal, Canada, May 5-9, 1996).