Polyvinyl Chloride (PVC)
The vinyl chloride monomer consists of a carbon-carbon double bond and a pendant chlorine atom and three hydrogen atoms (see Figure 7.10). This monomer polymerizes by the addition (free radical) polymerization method. This polymerization is done commercially using suspension, emulsion, bulk, or solution techniques and produces polymers that are non-stereospecific (atactic), although isotactic, syndiotactic, and atactic are all known under laboratory conditions. Vinyl chloride monomer (a gas) is a suspected carcinogen (may cause cancer) and has been shown to be toxic in large doses. Therefore, stringent limits have been established for exposure to vinyl chloride monomer, including exposure to the monomer during polymerization and a maximum permissible level of monomer residue in the polymer. These standards have been in place since the early 1970s in the United States. Therefore, polymerized PVC is free of monomeric vinyl chloride and is not considered a potential carcinogen. In fact, several food grades of PVC packaging material have been approved by the FDA. The polymerized product is usually a clear or white flake or powder, which is called polyvinyl chloride.
The presence of the chlorine pendant atom causes significant property changes in PVC compared to the polyolefins (PE and PP). The chlorine atom prevents close packing of the polymer and also provides a solvent sensitivity not seen in PE or PP because of the high electronegativity of the chlorine. The solvent sensitivity is so important that two types of PVC have been developed based on whether the PVC is modified with a solvent (plasticizer) or not. The unmodified PVC is called rigid PVC and the solvent-modified PVC is called plasticized PVC, or vinyl (see Photo 7.4).
An important property of PVC that is common to both rigid PVC and vinyl is flame retardance. When PVC burns, HCl gas is produced. This gas is more dense than air and therefore smothers the flame by excluding oxygen from the vicinity of the flame. As a result, PVC will burn with difficulty if an externally fueled flame (such as a bunsen burner) is in contact with the material but will extinguish if the fueled flame source is withdrawn. Materials with this characteristic are called self-extinguishing. Many applications for PVC, both rigid and vinyl, depend upon this self-extinguishing property. For instance, vinyl is used for wire and cable coating (such as Romex™) chiefly because it is self-extinguishing rather than because of its insulating capability, which is not as good as PE or PP.
Figure 7.10 Polymer repeating unit of polyvinyl chloride (PVC).
Photo 7.4 Various types of rigid PVC and plasticized PVC (vinyl). (Courtesy of Elf Autochem North America, Inc.)
Another inherent property of both rigid PVC and vinyl is thermal decomposition. When heated, even moderately, PVC tends to decompose by giving off HCI gas and forming crosslinks among the polymer chains when the HCI leaves. The color of the PVC generally changes to a yellow or, at advanced stages of decomposition, a brown. Once started, the decomposition process tends to cause other nearby Hand Cl atoms to combine, thus accelerating the decomposition. When sufficient decomposition sites have been formed within the molecule, the decomposition proceeds rapidly. This process is called autocatalytic decomposition. It is important, therefore, to prevent decomposition by minimizing the amount of the polymer's thermal exposure. In some polymers the effects of heat exposure accumulate over time. That is, even when the temperature is well below the normal point where decomposition occurs, repeated exposures will, nevertheless, cause decomposition. Polymers that are sensitive to accumulated heating are said to have a heat history. PVC is one of these polymers.
This tendency to thermal degradation is very important in the processing of PVC and complicates how PVC can be processed by heating. Molten PVC has a high viscosity that further complicates the problem created by thermal decomposition because PVC cannot be heated more to decrease the viscosity and allow easier processing. Processing would be extremely difficult except for several thermal stabilizers and lubricants that preferentially absorb thermal energy, reduce the viscosity of the melt, and thermally protect the PVC, as discussed in Chapter 5.
PVC is also sensitive to UV and oxidative degradation, through the extraction of HCI, indicated by yellowing. PVC can be protected by the addition of UV and oxidation stabilizers.
7.5.1. Rigid PVC
When compared with PE and PP, unmodified PVC is more rigid, stronger, and more solvent sensitive. The chlorine atom is approximately the same size as the CH3 group, as shown by the size comparisons in Figure 7.11. The size of the chlorine atom is sufficient to interfere with close packing and crystallization, resulting in a largely amorphous PVC polymer. Commercial PVC typically is less than 10% crystalline.
Even though PVC is largely amorphous, the size of the chlorine atoms causes significant intermolecular interference and the polarity of the Cl atom results in intermolecular attractions, thus increasing the tensile strength and modulus compared to PE and PP. The intermolecular interactions and general stiffness also increase the glass transition and melting point of PVC. The glass transition of PVC varies somewhat with the polymerization conditions used to make the polymer but is generally about 140° to 180°F (60° to 80°C), a temperature that is significantly higher than room temperature (73°F, 23°C). Since most applications for PVC are at room temperature, PVC is commonly used below its glass transition temperature, whereas PE and PP are used above their glass transition temperatures. It is not surprising, therefore, that unmodified PVC is much more rigid and brittle than the polyolefins in its unmodified (rigid) applications.
Rigid PVC is used in many applications where cost is a major factor. As a result, fillers are often used to reduce the cost. These fillers also add stiffness and may give some thermal stability benefit. The fillers usually reduce toughness. Typical fillers are talc, calcium carbonate, and clay. Impact modifiers can also be added to PVC so that the toughness will be improved. When properly formulated, rigid PVC can be processed by most conventional thermoplastic processing methods. In each of these processes, care should be taken to reduce the heat history of the resin. This is done by (1) processing at the lowest possible temperature, using additives that protect the resin, (3) utilizing equipment that gets good mixing without excess heating (such as twin-screw extruders), and (4) ensuring that the concentration of regrind in the mix is low so that the regrind material (which has a high heat history) is surrounded by virgin resin.
The metal dies and tooling used in the process should be carefully inspected because of the likelihood of corrosion from the HCl that is unavoidably given off by PVC, at least in small amounts, in any of the resin-heating processes. Corrosion can be very severe in some configurations and formulations. In these cases, replacement of molds, tools, fixtures, and screws should be considered and added into the overall cost of any PVC part. Corrosion-resistant coatings and metals are available to minimize the effects of HCI, but these also add cost to the tooling.
Figure 7.11 Size representations of common pendant groups in commodity polymers.
Extruded PVC products include house siding, pipe for sprinkler systems and electrical conduit, rain gutters, and window frames. Rigid PVC bottles can be made by blow molding, although plasticized PVC is more commonly used for bottles because it is less brittle. PVC is a commonly thermoformed material for low-cost, semirigid applications such as tote bins. Innumerable injection molded parts of rigid PVC are produced, especially when low cost, rigidity, and strength are needed. The sales of rigid PVC and vinyl are approximately equal.
The ability of PVC to be solvent and adhesively welded or joined is an obvious advantage over PE and PP, which generally must be welded with heat or some method that causes molecules to fuse together. Solvent welding employs traditional adhesives and is much easier to perform than the fusion or heat seal methods. A common example of the ease of joining PVC is seen in the solvent adhesive joining of sprinkler pipe. The materials to be joined, usually PVC pipe and a PVC fitting, are coated on the joining surfaces with the solvent adhesive. The parts are then placed together and allowed to dry. When the joint is dry (usually in about 24 hours, although 80% of the strength is achieved in 1 hour), the joint is often as strong as the surrounding plastic.
7.5.2. Plasticized PVC (Vinyl)
When plasticizer is added to PVC, the plastic is substantially more flexible than the rigid PVC just described. This plasticized PVC is commonly called vinyl. (The term vinyl is occasionally used to describe any molecule containing a carbon-carbon double bond and a noncarbon and non-hydrogen pendant group. This definition, though chemically correct, leads to confusion in plastics where the term has been associated so closely with PVC. Therefore, throughout this book, and in most common usage, the term vinyl, unless clearly indicated otherwise, will denote the plasticized form of PVC.)
The action of plasticizers was previously discussed in Chapter 5, where they were identified as chemicals that were added to plastics to soften them and add flexibility and elongation. Plasticizers act like a solvent for the plastic, where only enough solvent is added to cause some swelling, disentangling of the molecules, and some breaking of secondary intermolecular bonds, but where sufficient intermolecular interactions still exist that the material is not liquid. The plasticized material is generally a semirigid solid.
Adding plasticizers to PVC is very beneficial for many applications. The rigidity and brittleness of unmodified PVC can be substantially reduced by adding plasticizers, although this increase in flexibility is accompanied by a decrease in tensile strength. The impact toughness of the plasticized material increases initially and then decreases. The physical properties of a plasticized PVC suggest that the glass transition temperature of the material has been reduced such that at room temperature the material has passed from the rigid zone to the leathery or rubbery zone. This reduction in Tg has been confirmed by experimentation.
The plasticizer is usually infused into PVC flakes, granules, or particles before any thermal processing, rather than added to the melt. This pre-processing plasticization has the advantage of avoiding the difficult melting of PVC and its accompanying heat history. Since the plasticizers are solvents of the PVC, this infusion into the solid can usually be done with only a slight rise in temperature that swells the PVC and facilitates solvent entry into the plastic structure.
As discussed in Chapter 5, an ideal plasticizer imparts good flexibility to the polymer, is inexpensive and easy to add, does not add appreciably to the flammability of the plastic, is not toxic, is not extractable by sunlight, does not change the color of the polymer, and has low permeation rates so that it stays within the polymer. Few plasticizers meet all of these requirements, so products have been developed that can be sprayed onto vinyl and other plasticizable plastics to return plasticizer to the plastic. (Armorall™, an Armorall Corporation trademark, is one of these products.) These products seem to work well in practice. The difficulty with them is that they add the plasticizer to a formed part without the opportunity to heat the structure slightly to open it. To get the required penetration, the plasticizer molecule must be small. Consequently, the migration rate out of the vinyl is also high, and so repeated applications must be done to ensure continued plasticity.
The most common method of processing vinyl is to melt the plasticized pellets in traditional plastic-processing equipment. This can be done at lower temperatures than are used with rigid PVC because of the lower Tg and Tm that the plasticizer creates. This lower temperature processing reduces the heat history and extends the useful life of PVC over high-heat-history material.
Extruded vinyl is made into many parts including tubing (brand name Tygon™), which has wide application when flexibility, low cost, and optical clarity are important. Other extruded vinyl products include sheets for floor mats, fencing, house siding, and garden hose. In some of these products the improved flame retardance of vinyl over other plastics is an important characteristic. Extruded vinyl sheet can be readily thermoformed into products such as storage boxes, often with integral hinges. Blow-molded vinyl bottles are widely used where low cost, flexibility, and clarity are important. Bottles for cooking oil, bleach, and shampoo are some familiar products made of vinyl. Vinyl can also be made into blown film. It is widely used for shrinkwrap, food packaging, bags for blood plasma and other medical fluids, garment bags, and wall coverings. These film applications often rely on the ease of sealing vinyl with either solvent adhesives or with low-temperature fusion or ultrasonic sealing.
Some processing methods for vinyl do not require melting the vinyl plastic. One of the most common is adding sufficient solvent to the vinyl material that the vinyl dissolves or becomes suspended in the solvent. The resulting fluid-like material is called a plastisol or vinyl dispersion. The plastisol can be applied to other materials and then dried and fused (at moderately elevated temperatures) to form a soft vinyl covering. Many metal parts, such as racks for dishes and parts storage, are sprayed with or dipped in plastisol to provide corrosion protection and cushioning. Handles for screw drivers, pliers, and other tools are often made by dip coating in plastisol. Vinyl gloves are made by coating a mold (in the form of a hand) with plastisol and then drying. Vinyl can also be foamed, producing products such as carpet padding, weather stripping, and backing for various fabrics, and other sheet materials. Vinyl floor covering includes vinyl foam, a reinforcement layer, a printed film and backing material, as well as a vinyl wear surface.
The ability to process vinyl without heating extensively has given rise to an important processing method called calendering. This process involves the pressing of sheet materials together, such as vinyl onto cloth, by passing the materials through rollers. Vinyls are usually calendered by softening the polymer with high concentrations of plasticizer or solvent prior to pressing, a process that can be facilitated by slightly heating the materials. The calendering technique is used extensively for coating cloth to make vinyl seats and dashboard covers in automobiles and for general-purpose vinyl fabrics. Plasticizer that evaporates out of the vinyl fabrics (bleed-out) can sometimes be sensed as an oily residue on some vinyl fabrics and as an oily coating on the inside of automobile windows. This evaporation is highest when the temperature is high and so locations directly in the sunlight, such as the dashboard, are the most seriously affected. When the plasticizer migrates out of the vinyl, the vinyl material embrittles and often cracks. Recent formulation methods and plasticizer technology have dramatically reduced plasticizer bleed-out.
7.5.3. Vinyl Copolymers and Related Polymers
An important copolymer of PVC is made by combining vinyl chloride monomer with vinyl acetate monomer. The resultant copolymer is generally about 85% vinyl chloride. A major use of this copolymer is as a substitute for PVC homopolymer to improve the flexibility of finished vinyl flooring material. Other applications include many of those where PVC homopolymer is used but where rigid PVC is too stiff or where the plasticizer problems of vinyl (such as bleed-out) are unacceptable.
A plastic material that is chemically related to PVC is polyvinyliclene chloride (PVDC). The monomer for this plastic has a carbon-carbon double bond but has two chlorines on one of the carbon atoms. The polymer formed from the addition polymerization of this material has excellent barrier properties, especially against oxygen gas, and is used extensively as a food wrap. Polyvinylidene films have good cling properties, that is, a tendency to stick to itself, which is an advantage in food wraps. A common trade name for this material is Saran™, a trademark of Dow Chemical Corporation.