Understanding rigid PVC

Rigid PVC is a widely used polymer in finished products where good mechanical properties and durability are required. Thanks to its affordable price, it competes effectively against other plastics in the market. For its processing, it is essential to add basic additives such as thermal stabilizers and lubricants; otherwise, its transformation into a viable and functional product would be impossible.

In addition, it has certain dielectric properties and good flame resistance, which justifies its significant presence in hardware and construction markets. Also, thanks to recent developments in chemical additives, Rigid PVC is found in medical and even food applications. Although in the latter, the preference leans towards halogen-free plastics, it does not pose any harm to human health as long as the correct additives are chosen within its formulation.

Before we continue with the article, if you would like to know more about the additives we offer, you can visit our website at Chemical Additives from México or if you need pre-formulated compounds, you can visit us at Chemical Compounds.

1.- "What is rigid PVC?"

Polyvinyl chloride or PVC is a halogen compound due to the chlorine present at the molecular level. Thanks to its formulation capability, it can take on various forms according to customer requirements. This versatility, not offered by other plastics, has made it a highly resilient material despite failed attempts to displace it from the market. Its continued necessity is justified by its durability at a fairly affordable price.

To better understand this formulation ability, we can visualize the PVC particle as an irregularly shaped potato covered by a membrane and with a porous structure inside. This membrane, although it doesn't completely break until around 140°C, when exposed to heat, softens starting from about 80°C. This allows for the absorption of certain additives to modify its properties. To achieve optimal performance, the formulator needs to know the order of addition of these additives based on their polarity with the PVC resin and their melting point. These factors determine the temperature at which they should be added during the mixing process.

Rigid PVC, among commodity plastics, it contains only 43% derived from petroleum and 57% from brine or sodium chloride. The former represents a relatively low percentage compared to other plastics in the market. Its thermal degradation produces hydrochloric acid in the form of gas, which, although not a health risk for handlers under normal exposure, can cause irritation or respiratory issues over time. This gas serves as a catalyst in the thermal degradation process of PVC during its processing through a chain reaction or 'zipper,' and it is in our interest to reduce its emission using thermal stabilizers and minimizing processing temperatures wherever possible.

Rigid PVC and its characteristics

Rigid PVC it is a material that is not easy to process, as there is a belief that it can be produced for hours and days without the need for a production stop. This is completely false because despite the additives added that prolong its resistance to friction and temperature, the melt temperature tends to rise after prolonged hours of production, generating burnt particles or carbons that adhere to the equipment. Clearly, a better lubricated material will prolong the need for production stops for cleaning; however, it is inevitable.

Another characteristic worth mentioning is that Rigid PVC has a hardness in the range of 60 to 85 Shore D within commercial grades. Depending on its formulation, especially those with high impact resistance, it is considered a very viscous material and therefore requires higher processing temperatures compared to flexible PVC. In extrusion and injection, the processing temperatures typically range from 160°C to 185°C, although this depends on the equipment characteristics and the specific material. For instance, injection molding of rigid pipe fittings with a filled material is different from extruding a transparent profile with high impact resistance. There are variations not only in the process but also in the nature of the molten mass for processing.

Rigid PVC it has a softening temperature between 82-85°C, although additives, especially those with high polarity, tend to reduce this value significantly. Typically, formulators encounter the challenge that additives facilitating processing tend to sacrifice mechanical properties and vice versa. The solution lies in finding a middle ground where a product meets market demands (complying with the regulations governing the final product) and can be processed without major complications, striking a balance between these factors

It's important to clarify that the softening temperature (HDT) is not the same as the glass transition temperature (Tg, PVC 86°C). Although they are very close to each other, there are differences. Softening is a yielding term, where we perceive that finished parts undergo deformation, but there is no realignment of molecular chains. The Tg, or annealing temperature, is specific to a material with an amorphous structure, and beyond this point, we can improve the mechanical properties of a rigid material. This improvement occurs because the crystalline part of PVC, which is typically around 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 notice an improvement in tensile strength. Rigid PVC contrary to what happens in flexible PVC. It's important to remember that PVC, like any organic material, will tend to lose its properties if exposed to heat for too long, where the previous rule eventually becomes less valid.

Two terms that should not be confused with PVC are the glass transition temperature (Tg) and the melting temperature (Tm), with the latter being applicable to polymers with a crystalline molecular structure, as is the case with polyolefins. PVC has primary and secondary crystalline structures, with the primary structures undergoing annealing between 120°C-130°C. The secondary structures require a higher mass temperature and are the most difficult to eliminate, often causing lumps in finished products.

Although there are additives that can improve the Heat Deflection Temperature (HDT) of PVC, they come at a high cost, and their market availability is limited. Some examples include high molecular weight acrylic processing aids, which have the disadvantage of making the final product processing more challenging. The best recommendation is to avoid adding lubricants with high compatibility with PVC and opt for those that do not get absorbed into the molecule but merely coat it.

Regarding flammability properties, Rigid PVC in terms of flammability properties, PVC typically does not require additional additives to pass the most demanding tests, such as UL94. Due to its nature as a halogen compound, PVC has a high fire-extinguishing capability. However, it is important to exercise caution with added additives that could compromise or affect this property.

Additionally, Rigid PVC tends to exhibit low impact resistance properties, which worsen as we increase the amount of mineral fillers. While these fillers are functional for providing structural stability to PVC, in high quantities, there is a price to pay: equipment deterioration due to their abrasiveness, a reduction in impact resistance, and tensile strength. If we want to improve impact resistance, it is necessary to add specific additives, such as ABS, MBS, or others, which allow the distribution of impact energy throughout a finished product and thus prevent its fracture.

Rigid PVC by nature, it is rigid, odorless, transparent, and has a slight yellowish appearance. If any of these characteristics need to be altered, it is necessary to request your supplier to make adjustments to its formula. It does not have good UV resistance on its own, and it needs to be reinforced with antioxidants and UV agents if your finished product will be exposed to the elements.

2.- Processing of Rigid PVC

Rigid PVC has several processing methods which can be grouped into three main processes: injection, extrusion, and calendering. In any of its variants, we face a challenging task of producing finished parts that are useful for the next customer in the supply chain, presenting the minimum amount of waste, non-conforming parts, and rejections. The first thing to understand is that although Rigid PVC can be adjusted to meet the demands of the processing equipment (like a tailor-made suit), it clearly increases the cost of the raw material. There's a limit that few processors notice, which involves constant maintenance of the equipment.

There are those parts with a limited lifespan that are considered spare parts or consumables, an expense that any manufacturing company should include in its annual operating budgets. Failing to do this will generate problems that the raw material alone cannot solve, although the reader of this article will agree that the blame always ends up falling on the raw material.

Extrusion of Rigid PVC

The extrusion process involves melting Rigid PVC extrusion process involves melting the material and passing it through a screw, or twin screws, with the help of heaters that provide temperature. At the end of its path, it goes through a die and a mold that transform it into a continuous profile of a specific shape. Typically, we find extruders with a 1-meter long barrel with 4 heating zones, a head that, depending on its dimensions, can have 2 to 5 heating zones, 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 and limit the type of material they can process. Rigid PVC transparent in a conical twin-screw extruder may seem challenging without constantly experiencing complaints or rejections due to material degradation. This highlights the importance of conveying the operational and product needs of each company to machinery manufacturers.

In this extrusion process, a vacuum chamber is required to release the hydrochloric acid gases generated during the process. The absence of this can also lead to severe processing complications.

There are also screen changers, which, although more commonly used in flexible PVC, allow us to stop impurities, contaminants, or stains that may appear in our profile because they function as a trap. It is essential to be cautious with very tight screens because they can significantly increase back pressure and potentially damage our gears or bolts.

Purging for Rigid PVC

Many customers frequently ask us if in Chemical Compounds we offer purges for PVC. The reality is no, and the reason is as follows: a purge consists of a formulation with a high mineral content that allows it to carry away impurities (stuck carbon) inside the equipment.  As mentioned in this article, the use of fillers has an abrasive effect on the equipment, which increases the expense on spare parts, making the cost-to-benefit ratio of manufacturing them unfeasible.

A very common alternative is to use polyethylene purging since it is a plastic that can withstand process temperatures of 220°C-240°C, much higher than those of PVC. This allows the equipment to release any carbon buildup. Subsequently, a thorough cleaning is recommended since PE and PVC are not compatible with each other.

Rigid PVC Injection

When one needs to inject Rigid PVC, it is essential to be aware that this process involves a small extruder dedicated to melting the material and a second phase that consists of pushing the molten mass through a nozzle to then fill a mold, its veins, and cavities. In contrast to the problem faced in extrusion, where the challenge is to melt the material correctly to obtain a completely smooth piece, in injection molding, we usually have an excess of compression due to the nature of the process, constantly facing the risk of thermal degradation.

Every manufacturer's desire is to increase process productivity, but as with everything, this comes at a price. To reduce injection cycles in Rigid PVC we must establish much more extreme process conditions: 1) Raise temperatures to lower the mass viscosity to the point where the material does not burn. 2) Reduce loading time or back pressure, with material fusion being assisted by temperature. 3) Increase injection pressure to increase injection speed, with the risk of generating flash and reflux. 4) Reduce the pressure-holding percentage. 5) Reduce cooling times; avoiding a very cold mold that generates thermal shock that clouds the injected parts, but also not too hot that it does not allow the relaxation/fixation of the molecular chains, risking deformation. Finding the exact balance of all these variables is a challenging task that the operator must perform and fine-tune for each production line.

We wish to caution the reader that an injection machine, just like in extrusion, is not suitable for all plastics. In the case of extrusion, the L/D ratio, compression ratio, type of screw, and motor power are essential factors to determine whether it will allow processing rigid PVC. For the injection process, the injection capacity in tons is of vital importance, where a greater number of mold cavities require more power. There are also characteristics such as nozzle size, the loading screw motor, and the material being made of nitrided stainless steel to withstand hydrochloric acid. These are some of the elements to consider when performing the transformation of Rigid PVC. For all these reasons, before attempting to adapt a machine that not only does not guarantee its optimal performance, it is recommended to acquire an original factory-designed equipment initially intended for the desired production goals.

3.- Uses of Rigid PVC

The main uses Rigid PVC today are covered by the rigid pipe market and products intended for the construction industry, representing over 50% of the volume produced worldwide. However, some of its other applications include electrical components, medical devices, various types of profiles, among others.

The main limitation of using Rigid PVC for other applications lies in its softening temperature, as it cannot withstand temperatures above 70°C-80°C without undergoing some form of structural deformation. This issue is partially addressed by the use of CPVC, which can withstand a temperature of 100°C, which is why it is used in hot water pipes. However, for other components, sometimes it is preferred to switch to different plastics.

We recommend you visit our additives division at Chemical Additives from México, where we have recently added packages for calcium-zinc-based PVC pipes, offering an innovative proposal and the latest of interest to manufacturers. Not only does it improve the mixing process in pipe formula, but it also has less odor and allows for the production of a perfectly white tube with good physical properties compared to traditional additives.

4.- Advantages of Rigid PVC

• Affordable cost: clear, depending on its formulation, but it competes very well against other plastics.
• Versatility: as mentioned in the article, it can be formulated to provide specific characteristics to the product based on customer requirements.
• Halogen Compound: it easily extinguishes flames 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 aim to meet standards like Reach and FDA.
• Low odor: it depends on the additives used in the formula, but it can be done this way under the customer's requirements.

We hope this article is of interest to you. You can visit our website at Chemical Compounds or contact one of our expert engineers for further assistance.

Chemical Compounds