What Is PVC?
General Information: Although polyvinyl chloride (PVC) is the dominant member, the family of vinyls comprises a number of resins based on the vinyl radical, (CH2=CH-), or vinylidene radical, (CH2=C>).
Principal Uses of Polymers
Such polymers and their principal uses are:
- Polyvinyl acetate (PVAC) resin that is commonly used in latex paints, adhesives, surface coatings and textile finishings. It is also used as an intermediate in the manufacturing of other polyvinyls.
- Polyvinyl alcohol (PVAL) resin used in paper coatings, sizes, adhesives and textile finishings.
- Polyvinyl formal resin used in enamels for heat-resistant wire insulation.
- Polyvinyl butyral (PVB) resin, the adhesive interlayer used in the manufacture of safety glass.
- Polyvinyl fluoride (PVF) resin used as an outdoor weather-resistant coating.
- Polyvinylidene chloride (PVDC) has good resistance to moisture vapor and oxygen transmission and is widely used in food packaging film.
- Polyvinyl chloride (PVC) resin is the largest volume member of the vinyl family.
Its commercial value is greatly derived from the following characteristics:
1. Basic properties: chemically inert; water, corrosion and weather resistant; high strength-to-weight ratio; tough, dent-resistant; an electrical and thermal insulator; and maintains properties over long periods of time.
2. Process versatility; can be made in different form to permit processing on a wide variety of equipment; each form can be altered further by compounding to achieve particular properties in end products which range from soft to rigid in nature.
3. Properties can be provided at an economic cost. In the long term, PVC products are less energy-intensive on an installed basis than most conventional materials. Their light weight, insulating-and maintenance-free characteristics contribute to conservation of energy over the life of the product.
Four basic manufacturing processes for PVC resin exist. The extent to which each is used currently in the U.S. to produce about 6 billion pounds annually is as follows:
- 83% suspension
- 7% mass
- 8% emulsion/dispersion
- 2% solution
The primary reason for producing various types of PVC is to allow the resin to be processed in many ways, to take advantage of the inherent properties of PVC in many different applications.
Homopolymers comprise over 80% of the production in the U.S. Chemically, all PVC homopolymers contain about 56.8% chlorine with the balance being hydrocarbon. The three most important characteristics which affect the processing and use of specific resins are molecular weight, particle size and particle configuration.
- Higher-molecular-weight resins have greater strength, chemical and heat resistance but are difficult to process.
- Lower-molecular-weight resins process readily at some sacrifice in properties.
Most resins produced by the suspension, mass or solution process are in the range of 125 to 165 microns in diameter while those produced in the emulsion process for use in dispersions or latexes are less than 1 micron in diameter.
Particle configuration, particularly the degree of porosity, is important where liquid plasticizer must be absorbed in the preparation of dry powder blends which are to be processed directly in this form.
PVC is a thermally sensitive thermoplastic. Certain compounding ingredients must be added to the resin to permit it to be converted to an end product. Such ingredients are required for both processing and performance.
Heat stabilizers are common to all products for processing reasons and may be required for adequate performance in end applications. Other important additives used are lubricants, plasticizers, impact modifiers, fillers, biocides and pigments. The plasticizer is the major additive in a PVC compound (or formulation) since it imparts workability, flexibility, extensibility and resilience to a polymer system. Internal plasticizers are held in polymer systems by chemical bonds, while external plasticizers maintain their molecular identity in the polymer system and their comparability with it by virtue of forces such as hydrogen bonding and Van der Waals attraction.
When a plasticizer and a polymer are compatible, the two can fairly easily be mixed and fused into a coherent, homogeneous material and do not separate if a chemical or physical stress is imposed on the system.
A primary plasticizer for a given polymer has a high degree of compatibility at the projected use level. A secondary (or extender) plasticizer can only be safely used along with a substantial proportion of a primary plasticizer for the polymer system, to achieve satisfactory compatibility.
For reasons of compatibility, raw material cost, in-process behavior and performance in various end uses, the families of primary plasticizers for PVC are essentially limited to the following:
Dialkyl adipate esters, dialkyl azelates, some glycol dibenzoate esters, certain epoxy derivities including epoxidized soybean oil, epoxidized tall oil and some epoxy resins, glycollates such as butyl phthalyl, butyl glycollate, mellitates such as trialkyl trimellitates, a few phenoxy compounds, phosphate esters including triaryl, trialkyl and alkyl-aryl combinations, derivatives of ortho-phthalic acid with emphasis on dialkyl and alkyl benzyl o-phthalates, polyesters and various dibasic acids with glycols (e.g., adipic, azelaic and phthalic acids with various glycols terminated with a monofunctional compound), some pentaerythritol derivatives and various sulfonamides.
*Secondary (extender) plasticizers for PVC include the following:
Various aromatic and mixed aromatics aliphatic oils, chlorinated paraffins, some poly-alpha methylstyrene derivatives and inexpensive esters of high molecular weight alcohols and organic acids, which have marginal compatibility with PVC.
The resin selected and the method of combining it with some of the above ingredients into a compound depend both on the physical form desired for the type of processing intended and the properties required in the end product.
Vinyl compounds are processed or fabricated into useful products by two basic methods, melt and liquid processing.
In melt processing, rigid and flexible compounds in solid form are converted into melts by internal (shear) and external heating. This conversion is accomplished in several different types of thermoplastic processing equipment.
In extrusion, both single and multiple screw extruders are used to convert vinyl compounds into solid profiles, cellular profiles, pipe, blown film and flat sheet.
Calendering is used to produce flexible and rigid sheeting in the 2 to 35 mil thickness range. It is a large-volume processing method which requires high capital equipment costs compared with extrusion methods. Modern calender trains operate with computerized control systems to insure optimum quality. In calendering, it is necessary to preflux compound on Banbury/roll mill equipment at a rate to match the calender’s capacity. A short-barreled extruder downstream of the mixing equipment is often used to screen out any possible foreign contaminant which might damage the calender rolls.
Flexible products are based on medium to high-molecular-weight resins. Optimum processing with rigids is obtained with lower-molecular-weight PVC or with vinyl acetate copolymers.
* Not used by Dynamit Nobel of America (DNA) in Flexible Membrane Liner material products.
Specific Flexible Membrane Liner Information:
PVC liner materials are produced in roll form in various widths and thicknesses. Most liners are used as unsupported sheeting, but fabric reinforcement can be incorporated. PVC compounds contain 25% to 35% of one or more plasticizers to make the sheeting flexible and rubberlike. They, also contain 1% to 5% of a chemical stabilizer and various amounts of other additives. The PVC compound should not contain any water soluble ingredients. There is a wide choice of plasticizers that can be used in PVC sheeting, depending upon the application and service conditions under which the PVC compound will be used. Plasticizer selection is an extremely important aspect of a PVC liner material since the loss of plasticizer will result in a reduction of physical properties such as Low Temperature and Elongation. There are three basic mechanisms for plasticizer loss: volatilization, extraction and microbiologic attack. The use of the proper plasticizers and an effective biocide can virtually eliminate microbiological attack and minimize volatility and extraction. The PVC polymer itself is not affected by these conditions.
The principal reason for loss of plasticizer is by volatilization in the heat of the sun rather than solution in the waste fluid. Carbon black prevents ultraviolet attack but does cause the absorption of solar energy raising the temperature to a high level to cause vaporization of the plasticizer. A soil or other suitable cover material must be used to bury the PVC liner to protect it from ultraviolet exposure and virtually eliminates volatilization. PVC sheeting is not recommended for exposure to weathering and ultraviolet light conditions during its service life.
Plasticized PVC sheeting has good Tensile, Elongation and Puncture and Abrasion Resistance properties. It is readily seamed by solvent welding, adhesives and heat and dielectric methods.
PVC membrane liners are the most widely used of all polymeric membranes for waste impoundments. They show good chemical resistance to many inorganic chemicals; however, the inclusion of organic solvents may limit their applicability (consult manufacturer for specific applications). Special "Oil Resistant" (PVC-OR) grades of PVC are also available that possess a high resistance to oil and other organic hydrocarbon attack.
Dynamit Nobel of America (DNA) utilizes only first quality resin, primary plasticizers, stabilizers, biocides and other additives as described and discussed in detail under the headings General Information and Specific Flexible Membrane Liner Information.
If there is a need for any additional or more specific information regarding FML material products and their performance, please contact Mr. Richard H. Dickinson at DNA's home office in Rockleigh, N.J.
For more information, call 800-OK-LINER today!