Processing additives - A to Z.

Fatty acids and fatty acid derivatives.

Fatty acids

Fatty acids generally consist of a straight chain of an even number of carbon atoms with hydrogen atoms along the length of the chain and at one end of the chain, and a carboxyl group (-COOH) at the other end. It is the carboxyl group that makes it an acid. If the carbon to carbon bonds are all single, the acid is saturated; if any of the bonds are double or triple, the acid is unsaturated and is more reactive. Because they typically have limited solubility in rubber compounds, they act as effective processing agents.

Fatty acid derivatives

Fatty acid derivatives are mixtures derived from naturally occurring, long chain saturated or unsaturated carboxylic acids such as stearic, palmitic, oleic or linoleic acids (table 5). Today it is agreed that the most common used processing additive is stearic acid, also called octadecanoic acid. Stearic acid is a white crystalline, organic solid [CH.sub.3][([CH.sub.2]).sub.16] [Co.sub.2]H. Commercial stearic acid is a mixture of approximately equal amounts of stearic and palmitic acids, and small amounts of oleic acid. Stearic acid is used in many compounds as a cure activator. In addition to cure activation, the stearic acid also acts as a processing agent to improve mill and processing equipment release properties.

Table 5 -- unsaturated and saturated fatty acids

Unsaturated      Saturated

Crotonic acid    Butyric acid
Oleic acid       Caproic acid
Vaccenic acid    Caprylic acid
Linoleic acid    Capric acid
Linolenic acid   Lauric acid
Erucid acid      Myristic acid
                 Palmitic acid
                 Stearic acid

Zinc salts of unsaturated fatty acids

Another good example of this class is zinc salts of unsaturated fatty acids. These are useful as physical peptizers for natural rubber and other elastomers. If used in a high enough concentration, they can be used to stabilize thermal and oxidative chain scission while still offering the processing effects of fatty acids. The double bond acts to stabilize the free radicals generated during the chemo-mechanical breakdown. Rubber broken down in this manner usually will have a higher tear strength and resilience than rubber broken down by chemical peptizers such as thiophenols. Fatty acids and fatty acid soaps have a limited solubility in rubber compounds. This property is used to advantage in some rubber compounds when internal mold release is desired. However, when one wants to insure compatibility of the processing agent and eliminate bloom problems, fatty acid esters offer this property.

Esters

The esters, particularly those of multi-functional alcohols, are more compatible in the rubber compound than fatty acids or metal soaps. During esterification, compounds are formed, along with water, by a reaction of acids and alcohols (figure 1). The reaction between an ester and a metallic base is known as saponification (figure 2). When decomposition of an ester occurs upon its reaction with water, the ester is said to be hydrolyzed. The esters' physical form and melting point can be controlled by the degree of reaction. In addition, by suspending the esters on a fine particle silica, one is able to produce a free-flowing powder which is easily handled and readily incorporated into the rubber compound. In addition to processing effects, the esters will often improve carbon black and non-black filler incorporation into the rubber compound. Shown in figure 3 are polymer molecules with the dotted lines representing inter-molecular forces. When the polymer is at rest, the forces present maintain shape and form of the material. Under shear, during processing, it is necessary to overcome these forces in making the material flow or form. When fillers and reinforcing agents are added to the compound, the resistance to flow is increased. Processing agents, added to the system, act as lubricants and allow the system to flow more easily. Usually the higher the shear rate, the more pronounced the effectiveness of the processing agent. All natural fats and oils (other than mineral oils) and most waxes are mixtures of esters. A specific illustration of this effect is shown in the following example. A manufacturer of a high modulus, high hardness, nitrile rubber injection molded part was having difficulty getting the rubber compound to fill a mold. Several alternatives were examined. They included using a lower mooney viscosity rubber and using processing agents. Additional oil would have lowered hardness and modulus. Several compounds were prepared incorporating lower Mooney rubber and using processing agents in his existing compound. The results of capillary rheometer tests on the compounds are shown in figure 4. The low mooney rubber was the most effective at improving flow, but had an adverse effect on dynamic properties. The process agent, which was a fatty acid ester, improved flow without affecting physical properties. It was later evaluated in full production and it was confirmed to help flow and fill the mold.

[Figures 1-4 ILLUSTRATION OMITTED]

Wax additives

Wax additives are any of a class of pliable substances of animal plant, mineral and synthetic origin. To be called a wax, normally the compound must meet the following criteria:

* Solid at room temperature, but may vary from soft and plastic, to brittle and hard.

* Melt without decomposition above 40 [degrees] C.

* Macro- to micro-crystalline, translucent to opaque, not glassy.

* Low viscosity above the melting point.

* Consistency and solubility strongly temperature dependent.

* Polishable under slight pressure (ref. 2).

A variety of natural and synthetic materials meet these criteria and may be classified as shown in figure 5. Waxes are often added to rubber compounds to reduce tack and stickiness during processing of rubber compounds. These materials are inert and have limited solubility in the rubber compounds. It is for this reason the waxes also improve static ozone resistance as they bloom to the surface. As a result, one must be careful not to incorporate too high a level of these products, or metal adhesion and building tack can be adversely affected.

[Figure 5 ILLUSTRATION OMITTED]

Polyethylene waxes

Polyethylene waxes are synthetic waxes manufactured from the polymerization of ethylene. These are chemically similar to polyethylene resins, except they are much lower in molecular weight. The two key molecular properties which can be controlled by varying the reactor conditions are the molecular weight and the degree and type of branching. The higher the molecular weight, the higher the viscosity and toughness of the wax. Alternatively, as the density and crystallinity increase due to lower branching, the melting point and hardness of the wax increase. Polyethylene waxes can be compared with other hydrocarbon waxes as shown in table 6.

Table 6 -- hydrocarbon waxes

                              Paraffin   Micro-crystalline

Molecular weight              ~360-420   ~580-700
Branching                        Low        High
                                         Long chain

Melting point ([degrees] C)     50-70      60-90
Hardness (dmm)                  10-20       5-30
Crystals                        Large    Very small
                                plates     irreg.

Viscosity (cps)                  ~5         ~5
Toughness                        Low        Mod.
Flexibility                      Low        High

                              Fischer-     Polyethylene
                              Tropsch

Molecular weight              ~500-730     ~1,000-3,000
Branching                        Low         Low-high
                                           Short chain

Melting point([degrees] C)      95-110       90-120
Hardness (dmm)                   1-5         0.5-12
Crystals                      Very small   Very small

Viscosity (cps)                ~10           ~400
Toughness                       Mod.       Very high
Flexibility                     Low        Very high

Because of the versatility of the polyethylene polymerization, low molecular weight co-polymers with a range of vinyl comonomers can also be produced. Furthermore, polyethylene or co-polymer waxes can be modified after polymerization (post reacted) by oxidation, graft polymerization or various other chemical reactions. Some idea of the range of polyethylene waxes available is shown in table 7.

Table 7 -- property ranges for Polyethylene waxes

Product type    Density     Hardness
                (g/cc)       (dmm)

Homopolymers    0.88-0.93    0.5-98.0
Oxidized        0.85-1.00   <0.5-90.0

Copolymers      0.93-0.96    2.0-50.0
 Acrylic acid

Vinyl acetate   0.92-0.93    4.0-80.0

Product type       Dropping     Functionality
                    point
                ([degrees] C)

Homopolymers       90-120       None
Oxidized           88-140       Carboxylic acid
                                Acid num. = 5-41

Copolymers         76-105           5-18%
  Acrylic acid                  Acrylic acid

Vinyl acetate      75-102           6-26%
                                Vinyl acetate

Petroleum waxes

About 90% of all waxes used for commercial purposes are recovered from petroleum by de-waxing lubricating-oil stocks. Petroleum waxes are generally classified into three principal types: paraffin, micro-crystalline and petrolatum. All are used to some degree as processing additives. Paraffin wax is a mixture of solid straight-chain hydrocarbons ranging in melting points from 44 [degrees] C to 66 [degrees] C.

Paraffin wax was first produced commercially in 1867, less than 10 years after the first petroleum well was drilled. Paraffin wax precipitates readily from petroleum on chilling. Technical progress has served only to make the separations and filtration more efficient and economical. Purification methods consist of chemical treatment, decolorization by absorbents and fractionation of the separated waxes into grades by distillation, re-crystallization or both. Synthetic paraffin wax was introduced commercially after World War II as one of the products obtained in the Fischer-Tropsch reaction, which converts coal gas to hydrocarbons. Snow-white and harder than petroleum paraffin wax, the synthetic product has a unique character and high purity that make it a suitable replacement for certain vegetable waxes and as a modifier for petroleum waxes and for some plastics, such as polyethylene.

Micro-crystalline wax differs from paraffin wax in having much finer and less distinct crystals, and higher melting point and viscosity. In contrast to paraffin, micro-crystalline waxes may vary widely in character, depending on the crude-oil source and the method and degree of refinement. Some are ductile, like beeswax; others are hard and brittle; and still others crumble easily during handling. The melting point range is higher than that of paraffin wax, with commercial grades ranging from 63 [degrees] to 93 [degrees] C.

Lars C. Larsen is technical sales manager/ manager new business ventures for Struktol Co. of America in Stow, OH. He began his rubber industry career with BFGoodrich in 1968. He has been with Struktol since 1979.