How to Choose an Impact Modifier

Rigid unplasticized PVC offers high rigidity, chemical resistance, and durability. Without special modification, it is sensitive to impact loads, particularly at low temperatures and in the presence of stress concentrators.

To improve impact resistance, impact modifiers are added to the formulation. Their function is not simply to cushion an impact. The elastomeric phase changes the fracture mechanism of PVC, causing the material to dissipate energy through localized plastic deformation instead of rapid crack propagation.

In practice, a process engineer needs to answer three questions:

• which type of modifier is suitable for the specific application;
• which dosage provides the required impact strength;
• how the selected additive will affect processing and the other formulation components.

How the modifier works

As the additive particles are dispersed in the PVC matrix, they create areas where localized deformation begins under impact.

Depending on the structure of the compound, particle cavitation, shear deformation of the PVC matrix, microcrack formation, and other energy-dissipation mechanisms may occur. As a result, the impact energy is distributed through a larger volume of material, reducing the probability of rapid brittle fracture.

The product’s effectiveness therefore depends on more than its dosage. The following factors are important:

• glass transition temperature of the elastomeric phase;
• elastomer content;
• particle size and distribution;
• compatibility with PVC;
• processing temperature;
• service temperature of the finished product;
• filler content;
• stabilization, processing aid, and lubricant systems.

Core-shell systems have optimum particle-size and dosage ranges. Therefore, two products used at the same concentration in a formulation can provide completely different impact strength.

AIM, MBS, or CPE: which one to choose

Three main groups of impact modifiers are used for rigid PVC: acrylic AIM, MBS, and chlorinated polyethylene CPE.

The choice depends primarily on the application.

Acrylic impact modifiers (AIM)

Acrylic impact modifiers generally have a core-shell structure. The elastomeric acrylate core provides impact modification, while the rigid outer shell is responsible for compatibility with PVC, dispersion, and the processing properties of the powder.

The main advantage of acrylic systems is the combination of high impact strength and weatherability while retaining properties during long-term outdoor service.

This is why they are the primary choice for:

• window and door profiles;
• siding;
• facade and exterior technical profiles;
• fencing;
• other products requiring many years of service, low-temperature impact strength, and retention of appearance.

For a high-quality exterior profile, the selection process effectively starts with an acrylic system.

Another advantage of modern acrylic systems is their effect on processing. A properly selected product provides impact strength and accelerates fusion. It broadens the processing window, improves the surface, and increases extrusion stability.

MBS

MBS is an additive based on methyl methacrylate, butadiene, and styrene with a core-shell structure.

The butadiene elastomeric phase provides very high impact-modification efficiency. MBS is particularly suitable when impact strength must be combined with material transparency.

However, MBS has a fundamental limitation: low weatherability. It is therefore not used as the primary solution for products intended for long-term outdoor service.

The main applications of MBS include:

• transparent sheets;
• transparent profiles;
• packaging;
• interior technical products;
• fittings;
• other rigid PVC products without stringent weatherability requirements.

Thus, the high efficiency of MBS does not make it a universal solution. Its chemical nature becomes a limitation in outdoor applications.

Chlorinated polyethylene (CPE)

CPE is a fundamentally different type of modifying additive. It is a linear polymer in which the required dispersed structure forms directly during PVC mixing and processing.

Historically, chlorinated polyethylene has been widely used because of its availability and low price. Low cost remains its main advantage over modern high-performance acrylic systems.

However, these savings come at a technical cost.

Compared with a high-quality acrylic additive, this polymer generally has:

• lower impact-modification efficiency, particularly at low temperatures;
• poorer weatherability;
• later fusion;
• a narrower processing window;
• greater dependence of the result on processing conditions;
• a greater tendency to plate-out;
• lower surface gloss;
• greater product shrinkage.

Achieving comparable impact strength also often requires a higher dosage, so comparing the price per kilogram of the two products is meaningless.

Moreover, for certain products, such an additive should not be considered as an impact-modification option at all.

This primarily applies to products for outdoor use. These applications place stringent requirements on long-term weatherability, color retention, and surface quality. Resistance to impact loads at low temperatures and retention of properties after ageing are also important.

This category includes high-quality window profiles intended for long-term outdoor service. Using chlorinated polyethylene to reduce costs may result in a loss of impact strength after ageing, yellowing, and surface deterioration. A narrower processing window and additional processing problems are also possible.

Acrylic systems are used for such applications.

This material retains its place in less demanding opaque PVC products for which weatherability and long-term low-temperature impact strength are not critical requirements. In these cases, formulation savings justify the use of this additive.

Quick comparison

This table illustrates the key point: there is no universal solution for improving impact strength.

AIM is selected when product properties must be retained over many years of service. MBS is selected when impact strength and transparency are the priorities without long-term outdoor exposure. CPE is selected when the product requirements allow cost considerations to take precedence over maximum performance.

Why more modifier is not always better

Impact resistance does not increase linearly with elastomeric-phase content.

At a certain concentration, the fracture behavior transitions from predominantly brittle to ductile. Further increasing the amount provides progressively smaller gains in impact strength. At the same time, the formulation cost continues to rise, and the rigidity of the material may decrease.

Professional selection therefore means identifying the minimum dosage that consistently provides the required impact strength of the finished product, rather than the maximum dosage.

This distinction is particularly important when comparing AIM with CPE. A higher price per kilogram for an acrylic product does not necessarily mean a more expensive formulation because an efficient additive may provide the required result at a lower dosage.

Why the modifier cannot be evaluated separately from the formulation

Such an additive does not work in isolation.

The final result is influenced by:

• PVC type and K-value;
• calcium carbonate content;
• filler particle size and treatment;
• processing aid;
• stabilizer;
• internal and external lubricants;
• pigments;
• temperature and degree of fusion;
• extrusion conditions.

For example, increasing the content of a coarse filler may reduce impact strength and require a dosage adjustment. A change in the lubricant system may shift fusion behavior and change the effectiveness of the same acrylic system.

Therefore, two formulations containing the same amount of the same additive can produce completely different results.

Why fusion is important

Impact resistance depends not only on the dry-blend composition but also on how the blend is processed.

If the compound is insufficiently or incorrectly fused, even a high-quality additive cannot reach its full potential.

AIM, MBS, and CPE affect fusion differently. Modern acrylic modifiers generally promote earlier fusion and provide a wider processing window. CPE, by contrast, is more dependent on temperature and processing conditions.

Therefore, when switching from CPE to AIM, the process engineer can improve impact strength and often achieve a more stable extrusion process.

Does the lubricant system need to be changed when replacing a modifier?

Not necessarily, but it must be checked.

The first trial should be conducted with as few changes as possible: the new product is introduced into the existing formulation, after which the result and the behavior of the compound on the line are compared.

If fusion, load, pressure, surface quality, plate-out, or extrusion stability change, the next step is to adjust the lubricant and processing aid systems.

Thus, the transition to a new modifier can be carried out in two stages:

1. comparison in the existing formulation;
2. optimization of the dosage and formulation after the initial result.

This approach makes it possible to assess the product’s potential and then obtain maximum efficiency from it.

What to check during the first trial

The first trial does not require a complex research program.

It is sufficient to evaluate:

• impact strength;
• surface quality;
• fusion behavior;
• extrusion stability;
• plate-out;
• required dosage;
• economics of the final formulation.

For exterior products, both initial impact strength and the ability of the compound to retain its properties after long-term service are essential.

Solutions from Revada Plastic

Revada Plastic has many years of experience with impact modifiers and processing aids for rigid PVC. The company offers the Revamod range for different product and formulation requirements.

Revamod RM 21 is a highly efficient multifunctional impact modifier for profile extrusions, window profiles, pipes, and siding. It provides high impact strength at low temperatures, good fusion, and stable processing. RM 21 is used when outdoor reliability, including performance at sub-zero temperatures, is required.

Revamod RM 23 is an acrylic additive for the most demanding rigid PVC compounds. It combines high impact strength at low temperatures with weatherability, a wide processing window, high surface quality, and resistance to plate-out. This grade is particularly effective in window profiles and other products requiring mechanical performance, appearance, and stable long-term extrusion.

Revamod RM 41 is a CPE-based modifier. It is intended for formulations in which this additive is technically justified by the finished-product requirements and provides the required balance between properties and cost.

When selecting a grade, Revada Plastic evaluates the entire compound rather than the additive in isolation: PVC, filler content, stabilizer and lubricant systems, processing aid, processing conditions, and finished-product requirements.

For the first comparative trial, it is sufficient to provide the current formulation, the grade and dosage being used, and the required product characteristics.

The optimum impact modifier is determined neither by the lowest price nor by the highest value in a laboratory data table. It is the product that consistently provides the required impact strength, durability, and finished-product quality at the optimum cost of the final formulation.