Everything About Impact Resistance in Plastic Recycling

Why Impact Resistance Is the Heart of Recycled Plastic Quality

In the recycling industry, impact resistance is one of the first mechanical properties that customers look at when they decide whether to buy recycled materials.A plastic may have good tensile strength, acceptable melt flow, and decent color—but if it is brittle, it is immediately rejected for applications like:

• Automotive interior & exterior parts

• Consumer electronics

• Packaging

• Household products

• Construction profiles

• Industrial components

• Sports equipment

Impact resistance is directly linked to safety, durability, and performance. During recycling, however, most polymers experience a significant drop in impact strength—often by 40–80% depending on the material.

This blog post gives a complete, science-based explanation of:

• What impact resistance really means

• Why recycled plastics become brittle

• How different polymers behave

• How to improve impact resistance using additives and process optimization

• Industrial troubleshooting used in real recycling factories

• Advanced strategies (reactive extrusion, compatibilizers, tougheners, nanofillers)

By the end, you will clearly understand why toughness drops—and how to fix it.

1. What Exactly Is Impact Resistance?

Impact resistance is the ability of a polymer to absorb sudden energy without cracking or breaking.It is typically measured using tests like:

• Izod impact (ASTM D256)

• Charpy impact (ISO 179)

• Instrumented falling dart (ASTM D5420)

It is one of the best indicators of:

• Material ductility

• Energy absorption capability

• Interface strength in blends

• Quality of the polymer chain structure

• Level of degradation

From a molecular point of view, impact resistance depends on:

1.1 Molecular Weight

Longer chains → better entanglements → better toughness.During recycling, molecular weight drops, and so does toughness.

1.2 Crystallinity

• High crystallinity → stiff, brittle

• Low crystallinity → flexible, better impact

Recycling often increases crystallinity because degraded chains crystallize easier.

1.3 Morphology

In multiphase plastics (PP+PE, PA+PP, ABS+PC):

• Domain size

• Interface adhesion

• Phase dispersion

all determine toughness.

1.4 Rubber Phases

Materials like ABS or modified PP rely on rubber particles to absorb impact.These particles degrade during reprocessing.

1.5 Additives

Stabilizers, antioxidants, chain extenders, compatibilizers all influence toughness.

2. Why Does Impact Resistance Drop in Recycled Plastics?

Impact strength doesn’t just decrease randomly—it follows predictable, scientific mechanisms.

2.1 Thermal & Mechanical Degradation (the #1 cause)

Every time plastic is re-melted:

• Polymer chains break

• Molecular weight decreases

• Branches and entanglements disappear

• Polymer becomes less ductile

Example:

• Virgin PP impact strength: 20–40 kJ/m²

• After 3 cycles: <10 kJ/m²

This explains why recyclers always struggle to match virgin performance.

2.2 Oxidation

When polymers are heated in the presence of oxygen:

• Peroxides form

• Free radicals attack the backbone

• Chain scission accelerates

• Polymer becomes brittle

This is visible in:

• Yellowing

• Odor

• Increased melt flow rate

• Loss of impact resistance

PE, PP, ABS, and PA all degrade by oxidation.

2.3 Moisture & Hydrolysis (especially polyamides and polyesters)

PA6, PA66, and PET absorb water.In the extruder:

Water + heat → hydrolysis → chain breakage → low molecular weight → low toughness.

A typical recycled PA6 can lose 50–70% impact resistance without proper drying.

2.4 Mixed Polymers = Poor Compatibility

Most post-consumer and industrial waste is a mixture of polymer families:

• PP contaminated with PE

• PE contaminated with PP

• ABS mixed with PC

• PA6 mixed with PP

• PET mixed with PA or PS

• PS mixed with ABS

• PPE + PP mixtures in automotive waste

Each pair has different polarity, melt viscosity, and molecular architecture.This leads to:

• Phase separation

• Weak interfacial bonding

• Micro-cracks

• Brittle failure under impact

Example:PA6 + PP without compatibilizer → almost ZERO interfacial adhesionThis blend breaks like chalk.

2.5 Fillers and Reinforcements

Many recycled streams contain:

• Glass fibers

• Talc

• Calcium carbonate

• Wood fibers

• Paint and coating residues

• Adhesives

• Inorganic dust

These materials can:

• Increase stiffness

• Reduce elongation at break

• Create stress concentration points

When not controlled, they destroy toughness.

2.6 Multiple Thermal Histories

Recycled plastics have unpredictable histories:

• UV exposure

• Mechanical stress

• Heat aging

• Contamination

• Unknown additives

All of these contribute to low impact performance.

3. Understanding Impact Behavior in Different Recycled Polymers

Each polymer behaves differently when recycled.Here is a deep dive into how each one loses toughness and how to fix it.

3.1 Recycled Polypropylene (PP)

Problems:

• Severe chain scission

• Oxidation

• High PE contamination

• Loss of rubber particles (in impact copolymers)

Results:

• High MFI

• Low elongation

• Low impact strength

Solutions:

• Add PP-g-MA compatibilizer for PP/PE blends

• Add POE or SEBS to increase toughness

• Use antioxidants (AO-1010, AO-168)

• Use chain extenders to rebuild molecular weight

• Control melt temperature

Impact strength improvement: 2–4x with correct formulation

3.2 Recycled Polyethylene (PE)

PE is naturally ductile, but becomes brittle when degraded.

Problems:

• Oxidation

• Contamination with PP

• Gel formation

Solutions:

• AO stabilizers

• Blend with fresh LLDPE or mLLDPE

• Use elastomers (POE)

• Lower processing temperature

3.3 Recycled Polyamide (PA6, PA66)

PA is very sensitive to moisture and oxidation.

Problems:

• Hydrolysis during processing

• Incompatible mixtures with PP or PE

• Loss of amine end-groups

• High contamination from automotive waste

Solutions:

• Proper drying (80–90°C for PA6, 100–110°C for PA66)

• Epoxy chain extenders

• Maleic anhydride compatibilizers (PA-PE/PP)

• Tougheners like EBA, EMA, POE-g-MA

With proper formulation, recycled PA can reach:

• High toughness

• High elongation

• Class A impact performance

3.4 Recycled ABS and PC/ABS

ABS relies on rubber particles dispersed in a styrene matrix.

Problems:

• These particles break during reprocessing

• Oxidation

• Stress-cracking

• Moisture in PC/ABS

Solutions:

• SEBS toughening agents

• Styrene-based compatibilizers

• Stabilizers for thermal protection

• Drying at 90–110°C for PC blends

3.5 Recycled PET

PET becomes brittle due to:

• Hydrolysis

• Chain scission

• Increased crystallinity

Solutions:

• Multifunctional epoxy chain extenders

• Solid-state polymerization

• Nucleation control

• Compatibilizers for PET/PE or PET/PP blends

4. How to Improve Impact Resistance in Recycled Plastics (Complete Guide)

This is the most important part: industry-proven solutions that recyclers use to increase impact strength.

4.1 Using Compatibilizers (the most effective method)

Compatibilizers act like “glue” between incompatible polymers.

Common Systems:

Polymer Mix	Compatibilizer
PP + PE	POE, EBA, EMA, PP-g-MA
PP + PA	PP-g-MA
PA + PE	PE-g-MA
ABS + PC	Styrenic compatibilizers
PET + PE	EMA, EBA
PS + ABS	SAN-based compatibilizers

Benefits:

• Reduces domain size

• Improves interfacial adhesion

• Increases energy absorption

• Boosts impact strength significantly

Impact improvement: 50–300%

4.2 Adding Toughening Agents (Elastomers)

Rubber-modified plastics absorb impact by deforming instead of cracking.

Common tougheners:

• SEBS / SEBS-g-MA

• POE

• EPDM

• EBA (Ethylene Butyl Acrylate)

• EVA

• EMAA

Tougheners are especially effective in:

• PP

• PA

• ABS

• PC/ABS

4.3 Chain Extenders (Reactive Additives)

Chain extenders rebuild molecular weight especially in:

• PA

• PET

• PC

• PP

Examples:

• Multi-functional epoxies

• Carbodiimides

• Anhydrides

• Isocyanates

Effects:

• Higher molecular weight

• Lower MFI

• Better elongation

• More ductile failure

4.4 Antioxidants & Stabilizers

Stabilizers prevent oxidation and thermal breakdown.

Used in:

• PP

• PE

• ABS

• PA

• PET

Primary antioxidants → AO-1010Secondary antioxidants → AO-168Processing aids → phosphites, hindered phenolics

4.5 Moisture Control (critical for PA & PET)

Drying = non-negotiable.

If you do not dry PA or PET:

→ strong hydrolysis → immediate brittleness.

Drying guidelines:

• PA6: 80–90°C, 12–16 hours

• PA66: 100–110°C, 12–16 hours

• PET: 150–170°C, 4–6 hours

Vacuum drying or desiccant dryers give the best results.

4.6 Processing Optimization

Many recyclers underestimate this part. Correct processing can increase impact strength even without additives.

Recommendations:

• Reduce melt temperature

• Increase screw speed (shear thinning)

• Limit residence time

• Use nitrogen purge to reduce oxidation

• Avoid overheating in barrel zones

4.7 Nanofillers (Advanced Option)

Nanomaterials can enhance impact strength when properly dispersed:

• Nano-clays

• Modified graphene

• Core-shell nanoparticles

They improve toughness by changing morphology and absorbing stress.

5. Industrial Troubleshooting: Why Your Recycled Plastic Is Brittle

Below are real problems recyclers face, and their solutions.

Problem 1: Impact Strength Too Low in Recycled PP

Possible causes:

• High MFI → chain breakage

• PE contamination

• Poor dispersion

• Insufficient stabilizers

Fix:

• Add POE toughener

• Use PP-g-MA

• Add antioxidants

• Reduce processing temperature

Problem 2: Recycled PA6 Too Brittle

Causes:

• Hydrolysis

• Insufficient drying

• Contamination with PP or PE

Fix:

• Dry properly

• Add epoxy chain extenders

• Use PE-g-MA or PP-g-MA compatibilizers

Problem 3: ABS Fractures During Impact

Causes:

• Rubber domain breakdown

• Oxidation

Fix:

• Add SEBS

• Add antioxidants

• Lower processing temperatures

Problem 4: PET Very Brittle After Recycling

Causes:

• Crystallization

• Hydrolysis

• Chain scission

Fix:

• Dry at 160°C

• Add chain extenders

• Use nucleating agents

6. Why Improving Impact Resistance Increases Recycling Profitability

Better impact resistance means:

• Higher selling price

• Wider market (automotive, industrial)

• Better customer trust

• Less rejection and rework

• More circular loops before failure

• Higher sustainability metrics

A recycled material with high impact resistance is premium-grade.

Conclusion

Impact resistance is one of the most challenging—but also one of the most rewarding—properties to optimize in recycled plastics.By understanding:

• molecular degradation,

• compatibility issues,

• morphology,

• processing conditions,

• and the right additive systems,

recyclers can transform low-value mixed waste into high-performance, durable, tough engineering materials suitable for demanding applications.

This is how recycling becomes profitable, sustainable, and industrially competitive.Enhancing Polymer Performance with Additive Masterbatches