Rigid PVC is a widely used polymer in finished products that require good mechanical properties and long service life, and its affordable price allows it to compete effectively with other plastics on the market. For processing, it is essential to add basic additives, such as thermal stabilizers and lubricants; otherwise, transforming it into a viable and functional product would be impossible.
Additionally, it offers certain dielectric properties and good flame resistance, which explains its strong presence in the hardware and construction markets. Thanks to recent developments in chemical additives, rigid PVC is also used in medical and even food applications. While halogen-free plastics are preferred for the latter, it poses no risk to human health as long as the correct additives are selected in its formulation.
Before continuing with the article, if you need more information about the additives we offer, you can visit our website at Chemical Additives México, or if you require pre-formulated compounds, you can visit us at Chemical Compounds.
1. What is rigid PVC?
Polyvinyl chloride, or PVC, is a halogenated compound, due to the chlorine atoms present at the molecular level. Thanks to its malleability, it can be formulated into various shapes to meet customer requirements. This versatility, unmatched by other plastics, has made it a highly resilient material despite failed attempts to displace it from the market. Its continued use is justified by its durability at a relatively affordable price.
To better understand this formulation capability, we should visualize the PVC particle as an irregularly shaped potato covered by a membrane with a porous internal structure. This membrane, although it doesn't completely rupture until 140°C, softens at around 80°C when exposed to heat, allowing the absorption of certain additives and thus modifying its properties. To achieve optimal performance, the formulator must know the order in which these additives are added based on their polarity with the PVC resin and their melting point, which determine the temperature at which they should be added during the mixing process.
The Rigid PVC, Among commodity plastics, PVC contains only 431% petroleum derivatives and 571% sodium chloride brine, the former being a low percentage compared to other plastics on the market. Its thermal degradation generates hydrochloric acid gas, which, although not posing a health risk to processors under normal exposure, can cause irritation or respiratory problems over time. This gas acts as a catalyst in the thermal degradation process of PVC during processing, through a chain reaction or "zipper," and it is in our interest to reduce its emission by using thermal stabilizers and decreasing processing temperatures as much as possible.
Rigid PVC and its characteristics
The Rigid PVC It's a material that isn't easy to process, as there's a common misconception that it can be produced for hours or days without needing a production stoppage. This is completely false. Although the additives used extend its resistance to friction and temperature, the mass temperature tends to rise sharply with longer production runs, generating burnt particles or carbon deposits that adhere to the equipment. Clearly, a better-lubricated material will prolong the need for production stoppages for cleaning; however, this is unavoidable.
Another noteworthy characteristic is that rigid PVC has a hardness range of 60 to 85 Shore D among commercial grades. Depending on its formulation, particularly those with high impact resistance, the material is very viscous and therefore requires higher processing temperatures compared to flexible PVC. In extrusion and injection molding, processing temperatures generally range between 160°C and 185°C, although this depends on the equipment and the specific material. A clear example: injection molding of rigid pipe fittings, which uses a filled material, is not the same as extruding a transparent profile with high impact resistance, as there are variations not only in the process but also in the nature of the molten mass for processing.
Rigid PVC has a softening temperature between 82–85°C, although additives, especially those with high polarity, tend to reduce this value significantly. Formulators often face the challenge that additives facilitating processing may compromise mechanical properties, and vice versa. The solution lies in finding the optimal balance to produce a material that meets market demands (complying with regulations governing the final product) while remaining easy to process.
It is important to clarify that the softening temperature (HDT) is not the same as the glass transition temperature (Tg, PVC 86°C). Although very close, they have differences. Softening is a yielding process, where we observe that the finished pieces undergo deformation, but there is no realignment of the molecular chains. The Tg temperature, or annealing temperature, is characteristic of a material with an amorphous structure, and exceeding this point allows us to improve the mechanical properties of a rigid material. This improvement occurs because the crystalline part of PVC, which is typically 4%, tends to increase, reaching its limit at 20% when heated to 120°C. This is why, in aging tests using convection oven heating, we observe an improvement in tensile strength. Rigid PVC Unlike flexible PVC, PVC, it's important to remember that, like any organic material, PVC will tend to lose its properties if exposed to heat for too long, rendering the previous rule invalid.
Two terms that should not be confused with PVC are the glass transition temperature (Tg) and the melting temperature (Tm), the latter being applicable to polymers with a crystalline molecular structure, such as polyolefins. PVC has primary and secondary crystalline structures. Primary crystalline structures are those that undergo annealing between 120°C and 130°C. Secondary crystalline structures require a higher melting temperature and are the most difficult to eliminate, often causing lumps in finished products.
Although additives exist that improve the HDT (High-Density Thermal Transfer) of PVC, they are expensive and have limited market availability. Examples include high molecular weight acrylic processing aids, which have the disadvantage of hindering the final product's processing. The best recommendation is to avoid adding lubricants with high compatibility with PVC and instead opt for those that do not absorb into the molecule but merely coat it.
Regarding flammability properties, rigid PVC typically does not require external additives to pass the most stringent tests, such as UL94. By nature, as a halogenated compound, it has a strong self-extinguishing capacity; however, caution must be taken with added additives that could compromise or affect this property.
Additionally, rigid PVC tends to exhibit low impact resistance, which worsens as the amount of mineral fillers increases. While these fillers are functional for providing structural stability to PVC, high concentrations come at a cost: equipment wear due to their abrasiveness, and reduced impact and tensile strength. To improve impact resistance, appropriate additives—such as ABS, MBS, or others—must be incorporated to help distribute impact energy throughout the finished product and prevent breakage.
By nature, rigid PVC is rigid, odorless, transparent, and has a slight yellowish tint. If any of these characteristics need to be modified, it is necessary to request adjustments from your supplier. On its own, it does not have good UV resistance, so antioxidants and UV stabilizers are required if the finished product will be exposed to outdoor conditions.
2.- Rigid PVC Processing
Rigid PVC can be processed in several ways, which can be grouped into three main methods: injection molding, extrusion, and calendering. In any of its variants, we face the challenging task of producing finished parts that are useful for the next client in the supply chain, while minimizing scrap, non-conforming pieces, and rejects. It is important to understand that, although rigid PVC can be adjusted to meet the demands of the processing equipment—similar to a tailored suit made by a tailor, which naturally increases raw material costs—there is a limit that few processors recognize: the need for constant maintenance of the equipment.
There are parts with a limited lifespan that are considered spare parts or consumables, an expense that any manufacturing company should include in its annual operating budget. Failure to do so will create problems that raw materials cannot solve, although the reader of this article will likely agree that the raw materials are always ultimately blamed.
Rigid PVC extrusion
The extrusion process involves melting rigid PVC by passing it through a single or twin screw, assisted by heaters that provide the necessary temperature. At the end of the screw, the material passes through a die and head to form a continuous profile of a specific shape. Typically, extrusion lines feature a barrel about 1 meter long with four heating zones, a die head that may have between two and five heating zones depending on its size, and a cooling tank afterward to prevent shrinkage or deformation of the produced profile.
In Rigid PVC Single or twin screw extruders, conical or parallel, are used, which determine the degree of friction applied and limit the type of material they can process. Rigid PVC It seems difficult to operate a conical twin-screw extruder without constantly encountering complaints or rejections due to material degradation. This highlights the importance of communicating each company's operational and product needs to machinery manufacturers.
This extrusion process requires a vacuum chamber to release the hydrochloric acid gases generated during the process; otherwise, it can cause severe processing complications.
There are also mesh holders, which, although more commonly made of flexible PVC, allow us to stop impurities, contaminants, or stains that might appear on our profile, as they function like a trap. Care must be taken to avoid using very fine mesh, as this greatly increases back pressure and could damage our gears or bolts.
Drains for rigid PVC
Many clients frequently ask if Chemical Compounds offers PVC purging compounds. The reality is that we do not, and the reason is as follows: a purge is a formulation with a high content of mineral fillers designed to remove impurities (adhered carbon) from the equipment. As mentioned in this article, the use of fillers has an abrasive effect on machinery, increasing the cost of spare parts and making the cost-benefit of producing such purges unfeasible.
A very common alternative is to use polyethylene purge, as it is a plastic that withstands process temperatures of 220°C–240°C, much higher than those of PVC. This allows the equipment to release any carbon buildup. A thorough cleaning is then recommended, as PE and PVC are not compatible with each other.
Rigid PVC injection
When injecting rigid PVC, it is important to understand that the process involves a small extruder dedicated to melting the material, followed by a second phase in which the molten mass is pushed through a nozzle to fill a mold, including its runners and cavities. Unlike extrusion, where the challenge is properly melting the material to achieve a completely smooth part, injection molding typically involves excess compression due to the nature of the process, constantly posing a risk of thermal degradation.
Every manufacturer seeks to increase process productivity, but, as with everything, there is a cost. To reduce injection cycles in rigid PVC, much more extreme processing conditions must be established: 1) increasing temperatures to lower melt viscosity without burning the material, 2) reducing fill or backpressure time while assisting material fusion with heat, 3) increasing injection pressure to raise injection speed, with the risk of flash and melt-back, 4) reducing hold pressure percentage, and 5) shortening cooling times; avoiding a mold that is too cold, which could cause thermal shock and cloud the injected parts, but also not too hot, which would prevent relaxation/fixation of molecular chains and risk deformation. Finding the exact balance of all these variables is a challenging task that the operator must determine for each production line.
We wish to caution readers that, like in extrusion, an injection molding machine is not suitable for processing all plastics. In extrusion, the L/D ratio, compression ratio, screw type, and motor power are essential to determine whether rigid PVC can be processed. For injection molding, the injection capacity in tons is critical, as a higher number of mold cavities requires greater power. Other factors to consider when processing rigid PVC include nozzle size, screw motor specifications, and whether the material is made of nitrided stainless steel to withstand hydrochloric acid. For all these reasons, instead of attempting to adapt a machine that cannot guarantee optimal performance, it is recommended to acquire original factory equipment designed specifically for the intended production objectives.
3.- Uses of Rigid PVC
The primary uses of rigid PVC today are covered by the market for rigid piping and products intended for the construction industry, representing over 50% of global production volume. However, other applications include electrical components, medical devices, various types of profiles, and more.
The main limitation of using Rigid PVC For other applications, it's at a softening temperature, as it cannot withstand temperatures above 70-80°C without undergoing some type of structural deformation. This is partially resolved by using CPVC, which can withstand temperatures up to 100°C, hence its use in hot water pipes. However, for other applications, different plastics are sometimes preferred.
We recommend visiting our additives division at Chemical Additives México, where we have recently introduced packages for calcium-zinc-based PVC pipe. We offer an innovative solution featuring the latest developments of interest to manufacturers. Not only does it improve the mixing process in pipe formulations, but it also produces material with reduced odor and allows for perfectly white pipes with superior physical properties compared to traditional additives.
4.- Advantages of Rigid PVC
- Affordable cost: of course, depending on its formulation, but it competes very well against other plastics.
- Versatility: as already discussed in the article, it can be formulated to give specific characteristics to the product based on the customer's requirements.
- Halogen compound: Easily extinguishes the flame and demonstrates a good escape time.
- Adaptability: It adapts to the client's equipment and the specific requirements of each market.
- Low Toxicity: It all depends on the additives used in its formula, but the latest trends try to comply with standards such as Reach and FDA.
- Low Odor: It depends on the additives used in the formula, but it can be done this way upon customer request.
We hope you found this article interesting; you can visit our website at Chemical Compounds or contact one of our expert engineers for further advice.
Sincerely. Chemical Compounds