Reasons and solutions for adhesion in the production of soft PVC masterbatch

The core root cause of adhesion in the production of soft PVC masterbatch is the migration of plasticizers leading to surface adhesion, combined with factors such as insufficient cooling, poor formula compatibility, and improper process parameters, ultimately resulting in adsorption and adhesion between particles. Combining the characteristics of "high plasticizer content (50% -70%) and high viscosity of PVC carrier melt" in soft PVC masterbatch, the specific reasons can be disassembled according to the entire process of "formulation → mixing → extrusion → cooling → post-treatment", with clear logic and practical production.

I. Formula level: Core root cause (compatibility issues between plasticizers and additives)

The formula is the fundamental cause of adhesion, and the core is that the plasticizer is not stably locked by the PVC carrier. In addition, improper combination of additives directly leads to the surface of the masterbatch becoming sticky

1. Poor migration/compatibility of plasticizers (the core reason)

Improper selection of plasticizers: Low cost and easily migratory plasticizers (such as DOA, DOP) are selected, which have weak binding forces with PVC molecular chains and are prone to migrate from the interior of the carrier to the surface of the masterbatch, forming a continuous viscous layer that directly adheres to particles after contact;

Excessive addition of plasticizer: When the content exceeds 70%, the PVC carrier cannot completely encapsulate the plasticizer molecules, and the excess plasticizer will inevitably migrate to the surface, resulting in the overall stickiness of the masterbatch;

No anti migration agent added: Lack of epoxy soybean oil and polyester anti migration agents (2% -5%), unable to lock plasticizers through hydrogen bonds or chemical bonds, accelerating the migration process.

2. Insufficient or inappropriate smoothness/anti adhesion additives

Lack/insufficient amount of lubricant: lubricant such as erucic acid amide and oleic acid amide (0.3% -1%) is not added, or the amount is less than 0.3%, which cannot form a lubricating film on the surface of the masterbatch, resulting in high friction between particles and easy adhesion;

Anti adhesion agent not added/selection error: Physical isolation agents such as ultrafine silica and talc powder (1% -3%) were not added, or hydrophobic modified powders were used (which exacerbated adhesion after moisture absorption), which could not reduce the contact area between particles;

Excessive precipitation of additives: When the amount of lubricant and lubricant (stearic acid) exceeds 1%, it will instead precipitate from the inside, causing the surface of the masterbatch to frost and become sticky, forming a "secondary adhesion".

3. Conflict or excess of other additives

Improper selection of heat stabilizers: Some lead salt stabilizers and inferior calcium zinc stabilizers may react with plasticizers, damaging the bonding force between plasticizers and PVC and accelerating the migration of plasticizers;

Excessive total amount of lubricant: lubricants such as stearic acid and calcium stearate with a total amount exceeding 3% are prone to precipitate on the surface and increase viscosity.

II. Mixing process: uneven dispersion leading to local stickiness

Improper mixing process can cause "local enrichment of plasticizers", even if the overall formula is fine, local areas will still adhere due to excessive plasticizers:

1. Uneven mixing of plasticizers

Directly adding liquid plasticizers: without pre adsorption with PVC powder, the liquid plasticizers layer and aggregate during the mixing process, resulting in excessive concentration of plasticizers in some mother particles (locally>80%), causing local stickiness after migration and "point-to-point" adhesion between particles;

Insufficient mixing time/low rotation speed: When the mixing time is less than 15 minutes and the rotation speed is less than 500 r/min, the plasticizer is not evenly dispersed in the PVC molecular chain, forming a "local high concentration zone".

2. Moisture absorption of raw materials, damage and dispersion

The moisture content of raw materials such as PVC resin, fillers (such as calcium carbonate), and anti adhesive agents is greater than 0.1%. Moisture can disrupt the adsorption and bonding between plasticizers and PVC, resulting in uneven dispersion of plasticizers. At the same time, moisture causes the raw material particles to agglomerate, resulting in uneven internal structure of the masterbatch after mixing, and the surface is prone to sticking.

III. Melt extrusion process: Improper melt state exacerbates adhesion

Errors in extrusion process parameters can lead to uneven melting, accelerated volatilization/migration of plasticizers, and the inherent strong viscosity of the material strip

1. Excessive extrusion temperature or localized overheating

The barrel temperature is greater than 175 ℃ and the mold head temperature is greater than 180 ℃, exceeding the stable temperature range of plasticizers, causing a large amount of plasticizers to evaporate, and the residual plasticizers after subsequent cooling are prone to migration; At the same time, high temperature will damage the PVC molecular chain, produce viscous degradation products, and make the surface of the material strip sticky;

The temperature sensor malfunctioned, causing a sudden increase in local temperature of the barrel (>185 ℃), rapid migration of plasticizers in certain areas, and the appearance of "sticky spots" on the surface of the material strip.

2. Uneven melt mixing/excessive bubbles

The aspect ratio of the screw is less than 32 or the shear element is insufficient, the plasticizer and PVC are not fully integrated, and there are plasticizer aggregates in the melt. After extrusion, the surface of the material strip is uneven, and the contact area of the viscous area increases, exacerbating adhesion;

If the vacuum exhaust is not turned on or the vacuum degree is insufficient (>5Pa), the volatile substances and moisture of the plasticizer cannot be discharged, there are too many bubbles in the melt, the cross-section of the masterbatch has pores, the surface is uneven, and the particles are prone to "jamming and adhesion".

3. Uneven discharge from the mold head

The gap between the mold mouth is too small (<2mm) or uneven, the deviation of the material strip thickness is greater than 0.2mm, the fine material strip cools too quickly but may not be completely stable, the coarse material strip cools insufficiently, and the surface is still sticky. After mixing, the two adhere to each other.

IV. Cooling and granulation process: directly inducing adhesion (the most common direct cause)

Insufficient cooling and improper cutting parameters can cause the material strips that could have avoided adhesion to stick together directly, which is the most common problem in production:

1. The cooling water temperature is too high or the time is insufficient

Water temperature>25 ℃: The cooling speed of the material strip is slow, and the surface temperature cannot drop below 40 ℃ (still sticky to the touch). After cutting, the particle temperature is high and the surface becomes sticky, leading to direct adhesion;

Cooling time<30s: The material strip stays in the water tank for a short time, the internal plasticizer is not stable, the surface viscosity is not reduced, and it is still prone to adhesion after water discharge;

Not subjected to secondary air cooling: The material strip was not dried with a fan after being discharged from the water tank, resulting in residual moisture on the surface, and the temperature did not further decrease, exacerbating adhesion.

2. The cutting parameters do not match

Blunt cutting blade or excessive gap (>0.5mm): After cutting, there are burrs and wire drawing on both ends of the mother particle, which increases the contact area between particles. Wire drawing will cause particles to entangle with each other and directly adhere;

The traction speed and cutting speed are not synchronized: the length of the mother particles varies (deviation>1mm), with long particles prone to folding and short particles prone to stacking, increasing the probability of adhesion.

3. The strips come into contact with each other during cooling

There is no partition strip in the cooling water tank, and multiple material strips are wrapped and bonded together for cooling. When the surface viscosity does not dissipate, they come into direct contact, forming a "strip adhesion" that cannot be separated even after cutting.

V. Post processing and storage steps: subsequent adhesion triggers

Even if there is no adhesion during the production process, improper post-processing or storage can still lead to subsequent adhesion:

1. Incomplete secondary drying

After cutting, if the mother granules are not dried or not dried enough (temperature<60 ℃, time<1h), the surface moisture content is greater than 0.05%. The moisture makes the surface of the mother granules moist and accelerates the migration of plasticizers, causing adhesion during storage;

Excessive drying temperature (>80 ℃): On the contrary, it will cause the plasticizer inside the masterbatch to migrate again, resulting in a sticky surface.

2. Improper packaging and storage

The packaging timing is too early: the masterbatch is sealed before cooling to room temperature (>30 ℃), and the internal heat cannot be dissipated. The plasticizer continues to migrate, causing the masterbatch in the bag to adhere;

Packaging materials are not sealed: packaged in ordinary plastic bags without desiccant, the surface of the masterbatch becomes sticky after absorbing moisture, and the moisture in the air accelerates the migration of plasticizers;

Poor storage environment: Storage temperature>25 ℃, humidity>60%. High temperature and high humidity environment will significantly accelerate the migration of plasticizers, causing the masterbatch to gradually become sticky and adhesive.

The Core Logic of Adhesive Causes

The essence of the adhesion of soft PVC masterbatch is "the migration of plasticizers leading to surface stickiness", which is then amplified through three steps: "insufficient cooling (making the stickiness unable to dissipate), uneven dispersion (making the stickiness locally concentrated), and improper storage (making the stickiness further intensify)", ultimately forming adhesion. Among them, the selection/dosage of plasticizers and the cooling water temperature/time are the two most critical control nodes. Solving these two points can avoid more than 80% of adhesion problems.

Solutions

The core reasons for adhesion in the production of soft PVC masterbatch are plasticizer migration/surface stickiness, insufficient cooling, and poor compatibility with the formulation (soft PVC masterbatch contains 50% -70% plasticizer, which is prone to migrate to the surface and form a sticky layer). It is necessary to start from the entire process of "formulation optimization, process control, and post-treatment packaging" and solve them targetedly. Specifically, it can be divided into the following six key links.

I. Formula optimization: reducing stickiness and migration from the source (core and fundamental)

The core of the adhesion of soft PVC masterbatch is that the plasticizer is not stably locked by the PVC carrier. Formula adjustment needs to focus on "improving plasticizer compatibility+increasing surface smoothness":

1. Preferred plasticizers with low migration and high compatibility

Core principle: Avoid using low-cost and easily migratory plasticizers (such as DOA, DOP), prioritize environmentally friendly plasticizers with strong compatibility with PVC, and reduce surface stickiness from the source:

Recommended plasticizers: DOTP (dioctyl terephthalate), DINP (diisononyl phthalate), TOTM (trioctyl trimellitate), with a migration rate 30% -50% lower than DOA;

Prohibited/used with caution: DOA can be used in small amounts in short-term low-cost scenarios, but it needs to be accompanied by anti migration agents. It is strictly prohibited to use high migration plasticizers alone.

Plasticizer dosage control: Strictly control within 50% -70% to avoid excessive (>70%) use that may cause incomplete encapsulation of the PVC carrier and accelerate migration.

2. Add anti migration agents and compatibilizers

Anti migration agent: Add 2% -5% epoxy soybean oil and polyester anti migration agents to form stable hydrogen bonds with plasticizers and PVC, locking plasticizers from migrating to the surface;

Compatibility agent: Add 3% -5% PVC-g-MAH compatibility agent to enhance the binding force between plasticizer and PVC molecular chain, avoiding delamination and sticking.

3. Accurately add smooth/anti adhesion additives (key assistance)

Smooth agent: Select low precipitation and high temperature resistant amide additives (such as erucic acid amide and oleic acid amide), add 0.3% -1%, form a lubricating film on the surface of the masterbatch, reduce friction, and avoid adhesion;

Attention: The dosage should not be excessive (>1%), otherwise it may precipitate and cause frosting on the surface of the masterbatch, and subsequent products may become sticky;

Anti adhesion agent: Add 1% -3% ultrafine inorganic powder (silica and talc powder with a mesh size of 2000 or above), evenly disperse on the surface of the mother particles to form "physical isolation points", reduce the contact area between particles, and avoid adhesion;

Selection: Priority should be given to surface hydrophobic modified silica (such as KH-550 silane coupling agent treatment) to avoid secondary adhesion caused by moisture absorption.

4. Control the proportion of other additives

Lubricants: The total amount of lubricants such as stearic acid and calcium stearate should be controlled at 1% -3% to avoid excessive precipitation;

Heat stabilizer: Choose calcium zinc composite stabilizer (3% -5%) to avoid conflicts with plasticizers (such as some lead salt stabilizers that can accelerate plasticizer migration).

II. Mixing process: Ensure uniform dispersion of plasticizers to avoid local sticking

Soft PVC masterbatch adhesion is often caused by uneven mixing of plasticizers (local plasticizer concentration is too high, causing adhesion after migration), and the mixing process needs to be optimized:

1. Plasticizer pretreatment (liquid to solid, avoiding delamination)

Do not directly add liquid plasticizers: Pre mix the plasticizers with 5% -10% PVC powder for 10-15 minutes, allowing the plasticizers to fully adsorb on the surface of PVC particles and form a "solid paste", then add the remaining PVC carrier and functional components;

Mixing temperature: Control at 50-60 ℃ (below the volatilization temperature of plasticizers) to avoid premature migration of plasticizers caused by high temperatures.

2. Optimize the mixing order and parameters

Mixing sequence: PVC carrier → heat stabilizer (pre mixed for 5 minutes) → PVC paste with pre adsorbed plasticizer (mixed for 10 minutes) → lubricant+anti adhesive agent (finally mixed for 5 minutes);

Mixing parameters: rotation speed of 600-800r/min, total mixing time of 20-30 minutes, ensuring that the plasticizer evenly wraps around the PVC molecular chain without local enrichment.

3. Drying of raw materials (to avoid moisture absorption and increased adhesion)

All raw materials (PVC resin, fillers, anti adhesive agents) need to be dried in advance: PVC resin is dried at 80 ℃ for 2 hours, inorganic powders (silica, talc powder) are dried at 100-120 ℃ for 4 hours, and the moisture content is controlled below 0.05%;

Reason: Wet raw materials can damage the adhesion between plasticizers and PVC, accelerate the migration of plasticizers, and at the same time, moisture can cause the surface of the masterbatch to become damp, exacerbating adhesion.

III. Melt extrusion: Control temperature and melt state to avoid surface sticking

1. Precise temperature control to avoid plasticizer volatilization and PVC degradation

Temperature range: barrel 150-170 ℃, mold head 160-175 ℃ (5-10 ℃ higher than the end of the barrel), strictly prohibited from exceeding 180 ℃ (otherwise plasticizers will evaporate and PVC degradation will produce viscous substances);

Temperature uniformity: Regularly calibrate the temperature sensor to avoid local overheating of the barrel (local temperatures>180 ℃ can cause rapid migration of plasticizers).

2. Optimize screw and melt pressure

Screw parameters: Select screws with a length to diameter ratio of ≥ 32, equipped with strong shear elements (such as barrier type screws), to ensure full integration of plasticizers and PVC and avoid uneven melting;

Vacuum exhaust: Turn on the vacuum exhaust system (vacuum degree ≤ 5Pa) to remove plasticizer volatiles and moisture, reduce bubbles in the melt (bubbles can cause uneven surface of the masterbatch, increase contact area, and exacerbate adhesion).

3. Uniform discharge from the mold head (to avoid uneven thickness of the material strip)

Mold gap: Adjust to 2-3mm to ensure uniform diameter of the material strip (deviation ≤ 0.2mm), avoiding adhesion caused by rapid cooling of fine material strips and insufficient cooling of coarse material strips;

Surface of the material strip: If the surface of the material strip becomes sticky, the mold head temperature can be appropriately reduced by 5-10 ℃, or 0.2% -0.5% lubricant can be added.

IV. Cooling and granulation: key steps to directly avoid adhesion of the material strip

Insufficient cooling is the most direct cause of adhesion of soft PVC masterbatch, and targeted control of water temperature, cooling time, and cutting parameters is needed:

1. Cooling water temperature and time control

Water temperature: Strictly control at 15-20 ℃ (the optimal cooling temperature for soft PVC masterbatch) to avoid excessive water temperature (>25 ℃) causing the material strip to become soft and sticky on the surface, or insufficient water temperature (<10 ℃) causing the material strip to crack but still adhere;

Cooling time: The material strip stays in the cooling water tank for ≥ 30 seconds to ensure the stability of the internal plasticizer and the surface temperature drops below 40 ℃ (non stick to the touch);

Cooling method: Adopting "water tank cooling+air cooling secondary cooling", after the material strip is discharged from the water tank, it is blown by a fan for 10-15 seconds to further reduce the surface temperature and moisture content.

2. Particle cutting parameter matching to avoid particle adhesion

Granulator speed: synchronized with the traction speed of the material strip (traction speed 1-2m/min, granulator speed 500-700r/min), ensuring uniform length of the masterbatch (2-3mm) and avoiding overlapping caused by uneven length;

Cutting blade status: Use a sharp hard alloy cutting blade and polish it regularly (check every 8 hours) to avoid blunting the blade and causing burrs on both ends of the masterbatch (burrs will increase the contact area and exacerbate adhesion);

Cutting blade gap: Adjust the distance between the cutting blade and the mold mouth to be less than 0.5mm, ensuring that the incision is flat and free of wire drawing (wire drawing will cause particles to entangle and adhere to each other).

3. Material strip separation (to avoid adhesion during cooling process)

Install partition strips in the sink to separate and cool the strips, avoiding multiple strips from getting tangled together;

After the material strips are removed from the mold head, immediately use a traction machine to straighten them to avoid contact and adhesion between the material strips.

V. Drying and packaging: avoid adhesion during storage

If there is moisture or improper packaging on the surface of the cooled masterbatch, it will still adhere during storage and requires proper post-processing:

1. Secondary drying (removing surface moisture)

After cutting the mother granules, they are dried in a vibration dryer at a temperature of 60-80 ℃ for 1-2 hours to ensure that the surface moisture content of the mother granules is ≤ 0.05%;

After drying test: Touch the surface of the masterbatch with your hand, and there should be no moisture or stickiness to your hands (if it still sticks to your hands, you need to extend the drying time or add a lubricant).

2. Sealed packaging to isolate moisture

Packaging material: Use aluminum foil bags (dark, moisture-proof, sealed), add 5-10g desiccant (silica gel desiccant, avoid direct contact with masterbatch) to each bag;

Packaging timing: Pack the masterbatch immediately after cooling to room temperature (around 25 ℃) to avoid prolonged exposure to air and moisture absorption;

Storage requirements: Store separately (not mixed with hard PVC masterbatch or other moisture absorbing materials), with a storage environment temperature of 15-25 ℃ and humidity ≤ 60%, to avoid high temperature and high humidity accelerating plasticizer migration.

VI. Production environment control: Assist in reducing adhesion

Workshop temperature and humidity: Control the workshop temperature at 20-25 ℃ and humidity at 50% -60% to avoid high temperature and high humidity environments that may cause moisture absorption of the masterbatch and migration of plasticizers;

Equipment cleaning: Regularly clean the cooling water tank, tractor, and granulator (wipe the parts in contact with the material strip with anhydrous ethanol every 4 hours) to avoid residual plasticizers and materials sticking to the equipment and contaminating subsequent masterbatch.

The core logic of avoiding adhesion of soft PVC masterbatch is "locking plasticizer+reducing surface viscosity+sufficient cooling and drying":

In terms of formula: choose low migration plasticizer+anti migration agent+slip/anti adhesion agent to reduce stickiness from the source;

In terms of process: control the extrusion temperature, extend the cooling time, ensure uniform particle cutting, and avoid surface sticking;

Post processing: Thoroughly dry and seal the packaging to isolate moisture and high temperature environments.

By controlling the entire process above, the adhesion rate of soft PVC masterbatch can be controlled below 1% without affecting the flexibility and subsequent processing performance of the masterbatch.