Plasticizers for rubbers
Plasticizers for rubbers – The development of a new formulation for the manufacture of rubber artifacts, with characteristics that meet the specifications where the requirements are increasing, leads the “Formulating Technologist” to a constant search for information, with the maximum precision about each ingredient to be used in the composition.
When due to the needs of the application and use the artifact must be manufactured with some type of high-performance elastomer, the care in the choice of ingredients is even greater, because it is essential a thorough analysis of each property of the ingredients, such as physicochemical characteristics, compatibility with the other elements of the limit composition in the contents used, etc., data that if not carefully analyzed may cause failures with irreparable damage.
Plasticizers constitute a group of ingredients that, when used, play an important role in the good performance of a rubber artifact.
- As a processing aid:
- One of its main functions is to decrease the viscosity of the compound, thus increasing its plasticity, thus improving its processability at all stages of manufacture until vulcanization. This improvement in processability is due to an internal lubrication effect, which facilitates mixing, extrusion, calendering and molding operations;
- Improve the wetting of fillers and pigments, thus facilitating their incorporation in the mixing phase;
- Increase the flow velocity of the rubber compound, in operations such as extrusion and injection;
- Reduce temperature rise and decrease power consumption in the mixing process, which is due to its internal lubricant effect;
- Increase the stickiness level of the non-vulcanized compound (green tack). By improving the stickiness of the compound, it thus increases its adhesion power to the surfaces in contact, which is very important in operations of making rubber artifacts (e.g. tires, conveyor belts, footwear);
- As a modifying agent of the properties of the vulcanized:
- Decrease the hardness of the vulcanized, counteracting the increase in hardness that is conferred by the level of load or loads used;
- Improve the flexibility of the vulcanized;
- Shrink the module;
Decrease the rupture tension;
- Increase elongation at break;
- Increase tear resistance;
- Decrease the glass transition temperature Tg;
- Lowering Tg is equivalent to extending the service temperature range of the vulcanized artifact;
- Decrease the formation of electrostatic charges.
The choice of the most suitable type of plasticizer for a particular rubber compound should be guided by some criteria, such as:
- Compatibility between polymer and plasticizer;
- Volatility of the plasticizer during processing;
- Non-interference in the curing system of the compound;
- Resistance to extraction by solvents, oils, greases (vulcanized artef.);
- Low volatility at high temperatures (artef. vulcanized);
- Resistance to discoloration and tendency to staining;
- Handling safety and toxicity in the final artifact.
It is of paramount importance to choose the type of plasticizer that is perfectly compatible chemically with the polymer and other ingredients of the composition, because an inadequate choice may compromise the properties of the final product (usually a plasticizer incompatible with the polymer, tends to migrate to the surface of the artifact, being slowly extracted).
As a guideline, it is advisable to choose plasticizers with polarity similar to that of the polymer used in the composition.
A very important aspect to consider when adding plasticizer to rubber is its compatibility with it. If the plasticizer and rubber are compatible, a homogeneous compound will be formed, which will thus be maintained at all stages of the manufacturing process, after vulcanization and in service, over a wide range of temperatures. This compatibility must be observed for the dosage level used, a level that allows to obtain of a certain and desirable degree of effect on the various properties.
For this degree of compatibility to exist, it is necessary that the polarity of the rubber and the plasticizer are identical.
The polarity of plasticizers is a factor to take into account with their compatibility with the various types of rubbers. To obtain a good compatibility between the rubber and the plasticizer it is necessary that both have approximately the same polarity.
Aromatic, naphthenic, and paraffinic mineral (petroleum) oils are also indicated so that the respective polarity levels are positioned in relation to the various synthetic plasticizers.
A rubber compound during mixing and forming processing, typically reaches high temperatures (in some cases between 215 to 250 C).
The choice of plasticizer in this case is of great importance since certain types tend to evaporate at high temperatures, which will compromise the desired properties of the final artifact.
The choice in this aspect should take into account the temperature of the flash point of the plasticizer.
It is quite common to use vulcanized rubber artifacts, especially in seals, where the parts have direct or indirect contact with solvents, grease oils, and other chemicals, which tend to cause the extraction of the plasticizer contained in the rubber compound, consequently changing the initial properties.
Some types of synthetic plasticizers, such as polymers, or polymers of very low molecular weight and the same chemical family as the elastomer used in the compound, offer advantages of choice.
Vulcanized rubber artifacts are also widely used in constant working conditions at high temperatures, such as automotive parts (radiator hoses, gaskets, seals, etc.).
The action of continuous heat on the part may slowly cause the volatilization of the plasticizer if the choice of the plasticizer is incorrect.
Articles of vulcanized rubber, light in color, subjected to the action of sunlight, or under the effect of infrared or ultraviolet rays, may present some change in color. Basically, in this case, the determining factor is the choice of polymer. However, the use of inadequate plasticizers may further accentuate the problem.
Some types of plasticizers (especially aromatics, and petroleum derivatives) may cause staining on the color of the artifact, as well as on surfaces on which such artifacts have contact.
- Basically, we can classify the plasticizing ingredients into two groups:
Plasticizers of chemical action; and
- Plasticizers of physical action.
The plasticizers of chemical action are also known as peptizers (The product that makes it possible to reduce the viscosity of the polymer, improving its processing capacity, and facilitating the incorporation of the ingredients during the mixture.)
The plasticizers of physical action can still be subdivided into six basic categories, depending on their main action on the rubber compound, although all types have some effects in common such as the reduction of the viscosity of the compound, decrease in the hardness of the vulcanized artifact, reduction in the generation of heat during the mixing process and assist in the process of forming the artifact.
The basic categories of plasticizers are:
- Plasticizers of lubricating action;
- Plasticizers of diluent action;
- Plasticizers of encapsulating action (homogenizing);
- Plasticizers of solvent action;
- Plasticizers that improve Tack; and
- Specific plasticizers.
We can say that the plasticizers of these categories provide a certain degree of lubrication between the elastomer molecules, forming a thin film between them, which facilitates the sliding of one over the other. Basically, this category of plasticizers presents a mechanical interaction in rubber compounds.
In this category of plasticizers are classified petroleum derivatives, such as Paraffinic, Naphthenic, and Aromatic oils, as well as some resins of the same origin.
Synthetic plasticizers also belong to this category it is understood that, during the processing of the mixture, these plasticizers penetrate the intermolecular spaces of the elastomer causing a reduction of the interactive forces between the molecules facilitating the sliding of one on the other, and simultaneously create active interfaces of polymer – plasticizer – charges, which provides compatibility and dispersion.
Some synthetic plasticizers still cause modifications of the rheological conditions of the compound, improving the resistance to flexion at low temperatures.
Based on the use of plasticizer oils in rubber, we can initially separate into two applications, which are: Production of the elastomer and Processing of the compound.
The application of plasticizer in the production of elastomer, as a raw material, is called extender oils (SBR 1783/ Keltan 6675).
It is very common in the production (polymerization) of some types of rubber, such as certain degrees of SBR, BR, IR, and EPDM, to obtain polymers of excellent physical properties, but with high molecular weight, consequently very high viscosity, which makes it almost impossible to process mixing and forming with normal production equipment in rubber factories. Then, taking advantage of the properties of the polymer and seeking to adapt them to the normal means of processing (of the rubber industries), the producers of the polymers add defined amounts of extender oils thus forming “masterbatches”.
Paraffinic mineral oil is a petroleum-derived substance that has stable and resistant chemical alloys, which cannot easily be altered by chemical influences. These characteristics are justified by the fact that paraffins do not oxidize at ambient or relatively high temperatures.
These are paraffinic hydrocarbons containing carbon atoms combined with hydrogen by means of single bonds, offering linear or branched molecular chains.
The plasticizers (oils), highly paraffinic (those that have a predominance of paraffin molecules), present themselves as an almost transparent fluid.
They are considered non-staining, have low polarity, and are less volatile (more stable) at high temperatures.
Paraffinic plasticizers are more compatible with butyl rubbers and EPDM, presenting greater difficulty in incorporation into other types of elastomers.
As the amount of carbon atoms in the molecular chain of plasticizers increases, they become more viscous, heavy, and opaque, that is, the length of the molecular chain increases, and consequently, the molecular weight also increases.
Naphthenic oils have a molecular structure very similar to that of paraffinic oils, and in this case, too, carbon atoms have simple bonds. However, the arrangement in the chain tends to form cyclic rings.
Naphthenic oils have a good compatibility with the vast majority of common elastomers. Therefore, we can add higher contents (compared to paraffinic) to the rubber compositions.
Naphthenic plasticizers have a more opaque (translucent) coloration. The viscosity is slightly higher than that of paraffinic oils and is also considered to be non-staining in rubber artifacts.
Aromatic oils have a primary hydrocarbon structure containing six carbon atoms arranged in a ring shape, joined by single bonds and alternating double bonds.
The coloration of aromatic oils is quite dark. They are considered as staining plasticizers in rubber articles. The viscosity of aromatic oils is higher than that of paraffinic and naphthenic oils.
The existence of double bonds in the molecular structure of aromatic oils makes them very compatible with most rubbers that have unsaturated polymer chains. Aromatic oils are also widely used as extenders in the production of rubber. However, this type of plasticizer is less stable at high temperatures, volatilizing more easily. Therefore, they are usually not indicated for compounds that will be subject to the high temperatures of mixing or forming processing, or when the vulcanized artifact will work in the presence of a lot of heat.
Aromatic oils are also more easily extracted in solvent immersion tests, mainly chemical.
Synthetic plasticizers are widely used in polar elastomers of medium and high technical performance such as NBR, CR, CSM, CPE, Epichlorohydrins, and Polycrylics, mainly due to compatibility, because these families of elastomers are very sensitive to the chemical composition of plasticizers.
Synthetic plasticizers also offer very good properties to rubber compounds, both processing and vulcanized artifacts.
Typically, synthetic plasticizers are indicated when the vulcanized artifact must present superior resistance and work performance at low temperatures, or when the characteristics of resistance to extraction by solvents or chemicals is an important requirement in rubber parts, or when both properties (resistance to low temperatures and resistance to solvents) are required.
The polarity of synthetic plasticizers is given as a function of their dominant chemical structure, which is basically carbon-oxygen.
Synthetic plasticizers with long carbon chains are less polar. The production of synthetic plasticizers is the result of the reaction of several types of organic acids such as Anhydrides, Esters, Alcohols, Glycols, and Polyols.
Simply put, we can understand that synthetic plasticizers can be classified as:
- Monoesters – butyl-oleate; It is the result of monobasic acids with alcohol;
- Diesters – di-2-Ethylhexyl-adipate; is the result of acid-dibasic with alcohol;
- Glycols – triethylene glycol; is the result of monobasic acids with glycols;
- Triesters – tri-2-ethylhexyl-trimelite; It is the result of tribasic acids with alcohol (it can also be reaction of monobasic acids with glycerol);
- Polyesters – more commonly known as polymeric plasticizers; It is the result of the reaction of dibasic acids with glycols.
Usually, the plasticizers Monoesters, Diesters, Triesters, and Epoxidized have low viscosities, while the plasticizers Polyesters (polymeric) have higher viscosity.
Dioctyl phthalate is an oily liquid, usually transparent. It is soluble in most common organic solvents and is miscible in mineral oils such as ethanol and ether, although it is insoluble in water.
This product has low volatility, but care must be taken when storing it as it cannot be mixed with strong oxidants such as liquid chlorine and concentrated oxygen. Contact with open flames, heat, and sparks should be avoided. Requires adequate ventilation.
Dioctyl phthalate as the name implies, belongs to the group of phthalates. These are a group of chemical compounds, which are mainly used as plasticizers, as they increase the flexibility, elasticity, and elongation of rubber.
Our goal here was to gather some basic information about more of this ingredient used in rubber compounds that often, by inadequate choice, can compromise the desired characteristics, either of processing or of the vulcanized artifact.
We know that the universe of information about plasticizers is very wide. Therefore, close contact with the manufacturers of these products can help us in choosing the right condition for the condition we want.
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