Poly(vinyl chloride) (PVC) is a very unique material when compared to most other plastics. Its unique qualities make PVC one of the most widely used plastics worldwide. PVC is currently manufactured into an extremely wide range of products using numerous processing techniques. It is amazing to think that you could be standing over a PVC (vinyl) floor, looking through a window with PVC frames, while you open a faucet that is draining water through PVC pipes. You could even be looking at the neighbor’s house, which probably has PVC siding or a PVC fence. Most people do not realize it, but PVC is almost everywhere. PVC is abundant in construction materials like piping, siding, flooring, and wire; but it can also be found in a variety of other applications like packaging, household products, toys and medical devices. PVC can be formulated into materials ranging from hard and tough to soft and flexible. As shown in Figure 1, the possible ranges for the mechanical properties of this material can be extremely large depending on the formulation. So what gives PVC the ability to have such a vast range of properties? In order to understand this, one must understand the basics of PVC.
PVC is manufactured from vinyl chloride monomer via a free radical reaction to produce a polymer with a simple backbone structure as shown in Figure 2. The structure looks much like a polyethylene except a chlorine atom replaces hydrogen resulting in a chlorine content of approximately 55% by mass. The chlorine atom is much larger than a hydrogen atom which will greatly affect the stiffness of the backbone. Additionally, a halogen atom on the backbone will also provide a unique set of chemical resistance properties.
Like all plastics, PVC properties can be modified by changes in the chemical structure. Molecular weight (MW) is one of the important elements directly related to the molecular structure that will have an immense effect on the mechanical properties and processing behavior. Molecular weight values are controlled at the material synthesis level, but these are also affected during processing and while parts are in service. A reduction in molecular weight due to aggressive processing or chemical attack can lead to a brittle material behavior. Other changes that can be done directly to the PVC’s molecular structure include controlling the position of the chlorine atom or increasing the chlorine content. The latter results in another popular material, CPVC. However, this is a topic for another article.
Specific changes in the polymer’s chemical structure can be a means of affecting the properties of the material. However, it is usually not the preferred technique for controlling most properties of PVC. The use of additives is the preferred way to obtain PVC material with the best processing and performance characteristics. In general, modifying the polymer structure itself will only enhance certain properties, but not all the necessary ones to make reliable products. The use of additives, however, can enhance most of the properties of interest and usually at a fraction of the cost.
The truth that a lot of people do not know is that PVC on its own is of little use. Raw polymerized PVC is a material that is extremely difficult to process. It has poor heat stability and a high coefficient of friction that causes sticking of the polymer to metal surfaces of the processing equipment. If you were to process 100% PVC you would end up with overly brittle parts with very poor mechanical performance. So what makes PVC plastic such a versatile material? It is the combination of the numerous additives that make up a PVC compound.
PVC is truly a recipe of numerous additives such as fillers, stabilizers, lubricants and processing aids that are compounded together to create a plastic with unique processing and performance characteristics. The ability to use numerous individual additives provides the benefit of tailoring the material for a specific processing technique and application. In this article we will discuss several of the most commonly used additives for improving the mechanical performance of the final product. Additives specifically intended for affecting the processing of the material as well as processing effects will be discussed in the next article.
Common additives designed to improve multiple properties in the final product include plasticizers, UV stabilizers, impact modifiers, reinforcing agents and flame retardants. As evident from the chemical structure (Figure 2), PVC is a polymer with high polarity, which will make it compatible with multiple other high-performance polymers. Therefore, mixing other polymers into the PVC blend is a common technique for improving an otherwise poor material performance. Impact modifiers are an example of this.
Impact modifiers, as expected, will increase the toughness and prevent a brittle fracture response. Common examples of these modifiers include polymers such as ABS, CPE, EVA, and MBS. When mixing polymers such as these, other properties can also be improved. For instance, ABS will also improve the heat resistance while CPE, EVA, and MBS the processability. Many other polymers can also be added to the blend to improve other properties. For example, TPU can be mixed into PVC to increase resistance to abrasion and material elasticity. All these polymers need to be properly compounded within the PVC to result in a homogenous blend in order to optimize the material performance.
Many fillers and reinforcing agents are currently used with PVC. These may not only reduce cost but can increase the stiffness and enhance the shrinkage properties of the material. Common fillers include minerals such as calcium carbonate, kaolin and talc. Reinforcing agents can range from natural fibers to more complex reinforcements such as glass microspheres. In general, these agents can improve mechanical properties such as tensile strength, flexural modulus, and deflection temperatures.
Plasticizers are a unique set of additives that provide a whole new range of applications for PVC. We could write an entire thesis on the complexities of these additives and their interactions with the polymer. Different theories are used to explain the interaction between the plasticizer and the polymer. Whichever is more accurate, the end result is the same. Plasticizers interact with the polymer, resulting in a softening effect. Plasticizers are chemicals that show good affinity with the polymer. When PVC is plasticized, the molecular weight should not be affected. Plasticizers result in chemical effects that do not cause chemical attack (i.e. chain scission) but the affinity with the chemical used as plasticizer increases mobility.
Plasticization can be undesirable if the chemical has too much affinity (too aggressive) or in certain applications where the plasticizing chemical is unintentionally absorbed by the polymer resulting in a significant reduction in mechanical properties. The key characteristic of a plasticizer is that it needs to interact with the polymer-to-polymer molecular forces so that it reduces them, thus softening the material. However, it cannot be too aggressive, or it will otherwise solvate the polymer. Furthermore, the best plasticizers are those that can interact enough with the polymer chains so that they are not readily and easily diffused (leached) out of a part while in service.
Plasticizers are added to the polymer under certain specific conditions of heat and stress. Therefore, exposure to these conditions, as well as contact with unplasticized materials of similar affinity, can allow for the plasticizer to be diffused out of the part. Currently, polymeric-based plasticizers are commonly used when there is interest in limiting plasticizer diffusion since it is more difficult for the larger molecules to freely move. For example, for medical devices, certain phthalate free alternative plasticizers have been developed due to the increasing concerns on the possibility of plasticizers leaching out of the plastic and into the human body.
For PVC, the most common types of plasticizers used worldwide are phthalates. The most common types of phthalates used include DEHP (also known DOP), DINP, and DIDP. Stronger plasticizers that cause a solvation effect on PVC show high polarity and/or aromatic rings. When a plasticizer is absorbed by the polymer, the interaction with the polymer that leads to softening effects can be observed as reduction in the glass transition (Tg), modulus, and material density. This allows for an otherwise stiff and brittle material to behave in a flexible and moldable manner, which brings an altogether new set of applications for PVC material. Very common applications for flexible PVC include cable and wire, medical devices, tubing, and soft toys. One of the great benefits of most of these plasticizers is that they possess a whitish color which does not significantly affect the color of the material, allowing for transparent components to be manufactured.
Analysis techniques are available for examining composition and molecular changes in PVC. These techniques can provide valuable insight into the processing characteristics and final properties of the polymer.
As discussed in this article, the composition of a PVC formulation is critical to obtain the best performance for a given application. In Part 2 of this series, we will dive into processing, which is another critical aspect that directly affects the performance of PVC.
See the other installments in this series on PVC: