Lubricants

 In processing high melt viscosity polymers such as polyvinyl chloride (PVC) by extrusion, milling, calendering and injection molding, the shear forces applied cause excessive frictional heat which may lead to serious thermal stability problems. Another problem in processing PVC is to assure that the polymer releases from metal surfaces of the processing equipment. To solve these problems two types of lubricants are used.Lubricants which lower the melt viscosity and control frictional heat build-up are called internal lubricants while substances which promote release are called external lubricants. These materials are used in relatively small amounts since an excess will cause processingand stability problems and structural weakness in the ultimate product. In the processing of polymers such as PVC discrete particles are subjected to stress and heat until there is fusion of the discrete particles and a resulting loss of particle identity. An excess amount of an external lubricant will tend to coat the individual particle and while promoting a slippage between particles will delay fusion.

     The role of the internal lubricant is to reduce the internal friction within the polymeric melt, which includes reducing heat build-up when the polymer is subjected to stress.Because of the characteristic high melt viscosity of rigid PVC an internal lubricant is usually viewed as being necessary to improve flow properties. Their use will result in an economic advantage in that less work will be expended at a given set of processing conditions. In addition, improved product appearance usually results, particularyly with respect to improved surface appearance. An internal lubricant will promote fusion.

     Other distingguishing characteristics of internal and external lubricants are the effects they have on fusion time. Internal lubricants show no change in fusion time as the concentration of lubricant increases in the polymer system; external lubricants lengthen fusion time with increasing concentration.

      Some lubricants exhibit properties of both internal and external lubricants and are identified as internal/external. The degree of each type of lubricity imparted in a specific application is depending on the type and concentration of lubricant employed, the composition of the plastic system, the type of processing equipment, and the operating parameters of the processing system.

       In some instances, one encounters undersiable side effects in the use of lubricants, most notably in the reduction of heat stability which can lead to such major production problems as:

      Thermal degradation of the thermalplastic material within the extruder requires a halt of process operations for cleaning out.

       Recycling of materials is limited.

       The use of thermoplastic materials in light colored goods is limited.

     High level of expensive heat stabilizers may be required.

Plasticizers

Plasticizers are added to thermoplasts or elastomers to make them more flexible, improve processability, or allow them to foamed. Generally, plasticizers are low-molar-mass liquids, and only seldom are they low-or high-molar-mass solids. Elastomers are mostly plasticized with mineral oils: typical rubber tires, for example, contain about 40% mineral oil.Phthalic esters dominate plasticizers for thermoplasts, and here, di (2-ethyl hexyl) phthalate (“dioctyl phthalate “, DOP) is the most used.Polymeric plasticizers are only used in a relatively small number of cases. They are mostly polyesters or polyethers. High-molar-mass polyesters are used for polymer blends, but low-molar-mass polyesters are used as actual plasticizers. Since the latter are produced by polycondensation, they have a broad molar mass distrubution, and thus monomer and oligomer components. High monomer factions mean low polymer fraction, but quite high oligomer fractions. In such cases, they are called oligomeric plasticizers.

A distinction is also made between primary and secondary plasticizers. Primary plasticizers interact directly with the polymer chains, where sencondary plasticizers are actually only diluents for the primary plasticizers. For this reason, secondary plasticizers are also called extenders. This, depending on the polymers, a given plasticizer can act as either a primary or a secondary plasticizer. For example, heavy oils are extenders for PVC, but primary plasticizers for elastomers.

Eighty to eighty-five percent of all plasticizers are used to produce plasticized PVC. The phthalates are preferentially used to plasticize certain polyurethanes, polyester resin, and phenolic resin. Phosphaste esters are good plasticizers for melamine resins, unstaturated polyesters, phenolic resins, polyamides, and cellulose acetate. A total of about 500 different plasticizers are commercially available on the market.

Plasticizers incerease the chain segment mobility by different molecular effects. Polar plasticizers produce the gauche effect with polar polymer chains, that is, they increase the gauche conformation fraction at the expense of the trans conformations, and so reduce the mean rotational energy barrier. Acting as more or less good solvents, plasticizers dissolve helix structure and crystalline regions. In addtion, chain segments become more separated on account of the dilution effect. On the other hand, solvation does not increase chain mobility since a solvent sheath acts like a substituent and consequently increase the rotational energy barrier.

Because of the increased chain segment mobility, the glass transition temperatures, moduli of elasticity, tensile strengths, and hardness are decreased, whereas the extension at break is increased. The change in these parameters can thus be used as a macroscopic meaure of the effectivity of the plasticization. Of these parameters, only the glass transition temperature depends solely on the polymer chain mobililty, all other parameters contain contributions from other effects. Thus, measurements on plasticization effectively using glass transition temperatures, moduli of elasticity, tensile strengths, elongations at break, and hardness can not yield identical results.

To increase segment mobility, the plasticizer must be able to form a thermodynamically stable mixture with the polymer, that is, it must be compatible with the polymer, but solvents which are too good stifffen the chain by solvation. Thus, plasticizers must be solvents which are as poor as possible.

Stabilizers

Stabilizers provide protection against degradation caused by heat, oxidation, and solar radiation. Thus, when used in plastic compositions they may be classified as heat (or thermal) stabilizers, antioxidants and UV light stabilizers.

It is the role of heat stabilizers to prevent the polymer from degrading during the short period of high temperature (150℃ to 300℃) processing and to protect the finished plastic article against slow aging over longer periods at service temperatures.

Antioxidants inhibit or retard oxidative degradation at normal or elevated temperatures during processing, storage or service. Most polymers undergo some oxidation degradation, but hydrocarbon polymers are specially susceptible.Antioxidants, therefore, are generally added in small quantities.

Most plastics exhibit varying degrees of degradation upon prolonged outdoor exposure. Polypropylene, poly (vinyl chloride), polyethylene, polyesters, crstalline and high impact polystyrenes, and ABS are particularyly sensitive. Other plastics, particularly poly (methyl methacrylate) and the fluorocarbons, are much more resistant. To arrest or retard polymer degradation caused by the ultraviolet portion of solar radiation, plastic formulations contain UV absorbers. These are compounds such as substituted benzophenones, benzotriazoles and acrylonitriles that selectively absorb harmful radiation and convert it to heat energy.

Pigments such as titanium dioxide and zinc oxide are also used to protect plastics against the harmful effect of ultraviolet radiation.

They function by absorbing some UV radiation, but their ability to reflect radiation (heat as well as light) accounts for much of their effectiveness.In applications where colour is not a requirement, carbon black, which absorb UV light, is widely used as a very effective stabilizer (e.g. in black polyethylene). 

PVC Additives (2)

3. FILLERS

Essentially fillers are added to formulations to reduce costs, although they may offer other advantages – such as opacity, resistance to blocking, reduced plate-out, improved dry blending. On the other side, fillers can reduce tensile and tear strength, reduce elongation, cause stress whitening, reduce low temperature perforance.

The most common fillers used with PVC are calcined clays, and water-ground and precipitated calcium carbonates of particle size around 3 micrometers. Other fillers are silicas and talcs.

4. LUBRICANTS

     These materials are of prime importance in PVC processing. They are decribed below.

      ⑴ Improve the internal flow characteristics of the compound.

      ⑵ Reduce the tendency for the compound to stick to the process machinery

      ⑶ Improve the surface smoothness of the finished product

      ⑷ Improve heat stability by lowering internal and/or external friction

          Examples of lubricants, with which you may be familiar, are stearic acid, calcium stearate, Wax E, polythylene AC 617.

5. PROCESSING AIDS

These may be regarded as low-melt viscosity, compatable solid plasticizers. They are added to lower processing temperatue, improve roll release on calenders, reduce plate-out, promote fusion.

They are usually added at concentration of 5.0%. The most widely used processing aids are acylic resins of which acrylic K 120N is an example.

6. OTHER ADDITIVES

     There are  several other addtives which we will list and comment on briefly.

⑴ Impact Modifiers: These are usd in rigid vinyls to improve impact resistance. These are usually acrylic or ABS polymers used at 10-15 phr levels. Examples are Kureha BTA 111, Blendex 301.

⑵ Light Stabilizers: for resistance to ultraviolet radiation. They are used in low concentrations 0.5-1.5 phr.

⑶ Flame Retardants: PVC is inherently self-extinguishing However, the plasticizers and additives are not. Therefore, flame retardants are added. The most widely known one is antimon tri-oxide.

⑷ Anti-Static Agents

⑸ Fungicides: Vinyzene BP-5

⑹ Foaming Agents: Chemicals that decompose at predetermined temperature to produce a certain volume of gas within the molten vinyl and thereby create foam.

⑺ Colorants: Both pigments and dyes can be used. However, dyes, which are soluble organic substances,are used sparingly de to their tendency toward migration and extract ability. Heat resistance of colorants must be carefully evaluated.

    In summary, we have seen that a vinyl compound consists of following components: PVC resin, plasticizer, heat stabilizer, lubricant, special additive, colorants.

PVC Additives (1)

It should be noted that PVC resin, of themselves, are of no practical use. When fused, they are hard, brittle compounds. Their inherent limited heat stability make any type of processing difficult if not impossible. Therefore, in order to produce a useful product other ingredient are added to the PVC resin for the purpose of : increasing flexibility, providing adequate heat stability, improving processability, imparting aesthetic appeal.

Let’s consider these ingredients in some detail.

1.     PLASTICIZERS

  Plasticizers are low boiling liquids or low molecular weight solids that are added to resins to alter processing and physical properties. They increase resin flexibility, softness and elongation. They increase low temperature flexibility but decrease hardness. They also reduce processing, temperatures and melt viscosity in the case of calenering.

   Plasticizers fall into two categories based on their solvating power and compatibility with resins.

A.    Primary Plasticizers: are able to solvate resins and retain compatibility on aging. Samples of these would be:

DOP      Dioctyl phthalate

S-711     Di(n-hexyl; n-octyl; n-decyl) phthlate (linear)

DIDP      Di-iso decyl phthate

B.     Secondary Plasticizers: are so defined because of their limited solubility and compatibility and are, therefore, used only in conjunction with primary plasticizers. The ratio of primary to secondary depends on the type and quantity of the particular plasticizers. Secondary plasticizers are used to impart special properties such as:

-low temperature flexibility  DMODA (di-normal octyl decyl adipate)

                         DOZ (di-octyl azelate)

                         DOA (di-octyl adipate)

-         Flame retardance  Reofas 65 (tri-iso propyl phenyl phosphate)

-         Electrical properties t  tri-mellitates

-         Cost reduction        Cereclor, chlorinated parafins

In a separate category are the polymeric plasticizers. These are long chain molecules and are made from adipic, azelaic, sebacic, acids and propylene and butylene glycols. The efficiency of polymerics is poor but volatility and migration are superior. An example of a polymeric plasticizer is Paraplex G-54.

The characteristics sought in plasticizers can be summarized as follows:

⑴ efficiency- This is the level or concentration needed to give a stated hardness, flexibility or modulus.

⑵ the effect on low temperature flexibility.

⑶ solvating power: This influence the fluxing rate of the compound at a given temperature or at a minimum fluxing temperature.

The fluxing rate relates directly to processing time.

Performance: This relates to volatility, extraction resistance, compatibility.

2.     HEAT STABILIZERS

   The chief purpose of a heat stabilizer is to prevent discoloration during processing of the resin compound. Degradation begins with the evolution of Hydrogen Chloride, at about 200 ℉ increasing sharply with time and temperature. Color changes parallel the amount of degradation running from white to yellow to brown to black. Therefore, the need for heat stabilizers.

   The most effective stabilziers have been found to be:

   ⑴ Metal soaps: Barium-cadmium solids and liquids: Mark 725, Mark 311

   ⑵ Organo tin compounds: octyl tin mercaptide: Mark OTM

   ⑶ Epoxies: epoxidized soya oil (G-62)

   The above are most likely most effectively only when used in combination (synergism).

   What are some of the criteria in choosing a stabilzier system?

⑴ The ability to prevent discoloration

⑵ The amount of lubrication involved. In calandering this can be of critical importance. Mark 725 has low lubricating effect while Mark 311 contributes high lubrication effect.

⑶ Plate-Out- a potential side-affect of processing and has been linked to certain barium-cadium stabilizers.

⑷ Compatability with the resin system- for obvious reasons.

⑸ Resistance to sunlight staining: atmospheric discoloration.

What is PVC?

Inherent Properties of PVC

Containing 56.5% chlorine and 43.5% ehylene from petroleum feedstrocks, PVC is much less dependent than most other thermosplastic resins on the fluctuations of supply and demand of the petroleum industry. Its chlorine content is derived from table salt!. PVC’s chlorine content provides inherent flame & fire retardancy. Other additives (plasticizers, modifying resins) may burn, but PVC will not support combustion on its own.

PVC is regarded as perhaps the most versatile thermoplastic resin, due to its ability to accept an extremely wide variety of additives: plasticizers, stabilizers, fillers, process aids, impact modifiers, lubricants, foaming agent, biocides, pigments, reinforcements. Indeed, PVC by itself cannot be processed. It must have at least a stabilizer, a lubricant, and if flexible, a plasticizer present.

PVC products can run the gamut from a wiggly fishing worm to a high impact computer housing, pipe, windows and fencing, and all in between. Clear or opaque, flexible PVC applications (flooring, automotive, wire & cable) donimated the earlier years (40’s, 50’s, 60’s), but with the advent of reciprocating screw injection molding and twin screw extrusion in the 60’s, rigid PVC began to flex its muscle in pipe fittings, siding, electrical junction boxes, fencing, docking, to the point today where rigid PVC applications account for about 70% of all PVC processed!

Physical properties of course will vary widely depending on types and amounts of additives chosen. Based on cost/performance, many consider rigid PVC to be “the poor man’s engineering resin”!

PVC has a unique degradation sequence. Unlike most other polymers that exhibit mainly oxidative degradation with peroxide formation and chain scission, protected by antioxidants, PVC (while also undergoing oxidative degradation) has a nasty habit of releasing HCl under heat and shear of processing—an “unzippering effect” that rapidly progresses to catastrophic charred blackening if left unchecked. The art and science of stabilization—a whole industry sector—has developed very effective protective stablizer additives to retard this type of degradation. This HCl elimination is most likely to start at a “weak link” site—typically a chlorine on a carbon at a branching site in the chain. The result is a series of alternating ( or conjugated) double bonds, and the onset of visible discoloration (yellowing) has been peged at 7-8 conjugated double bonds. However a UV black light can see early degradation at 3-4 double bonds before it becomes visible to the eye.

Volumetric Feeder For Extrusion Process

Volumetric feeder provides low cost and easy option in batch or continuous applications, especially when the bulk density of the fed material is consistent or when precise feed accuracy is not so important for the quality of end product. Frequently volumetric feeder is used in gain-in-weight batch application since higher accuracy can be achieved. When process requirement is more demanding, volumetric feeder with gravimetric scale and loss-in-weight control can be used.

Our volumetric feeder can provide reliable and stable performance. Single screw feeder is the best choice for free flowing materials, while twin screw feeder is the the best solution for difficult-handled materials. We have a full range of volumetic feeders with a number of models and different-sized hoppers to meet your specified requirements.

Twin-screw volumetric feeder is applied to provide accurate volumetric feeding for poor flowing powder, fiber and flake materials.

All parts which contact fed material are made of stainless steel. It is easy to disassembled for cleaning or changing parts. The special bowl-shaped hopper makes material flow from all directions into the screw evenly. It can also avoids material build-up and make feeding more accurate.

Two types of hopper, symmetrical or asymmetrical, can be chosen.

The standard construction material of hopper is SS304. SS316 and SS316L are optional to be used.

For poor flowing materials which tends towards bridging, a vertical agitator is optional to add.

For different environments, different construction materials of screw can be used. The screw materials can be SS304, SS316L, copper, carbon steel and Hartz alloy.

For more information, please visit ELH Machinery.