Modified PBT (Polybutylene Terephthalate): Common Problems and Solutions

Common Problems and Solutions for Modified PBT 

 

Polybutylene terephthalate (PBT) possesses excellent comprehensive properties, such as high crystallinity, rapid molding capability, weather resistance, low coefficient of friction, high heat distortion temperature, good electrical properties, excellent mechanical properties, fatigue resistance, and ultrasonic welding capability. However, it has low notched impact strength, high molding shrinkage, poor hydrolysis resistance, and is susceptible to halogenated hydrocarbon corrosion. After glass fiber reinforcement, the inconsistent longitudinal and transverse shrinkage rates of the product can easily cause warping.

 

I. Notch Sensitivity

 

The benzene ring and ester groups in the PBT molecule form a large conjugated system, reducing the flexibility of the molecular chain and increasing molecular rigidity. Furthermore, the presence of polar ester and carbonyl groups increases intermolecular forces, further enhancing molecular rigidity and resulting in poor toughness.

 

Solutions:

 

1. Polymerization Modification: Polymerization modification involves introducing new flexible segments into the PBT molecule during polymerization through copolymerization, grafting, block polymerization, crosslinking, etc., thereby giving it good toughness.

 

2. Blending Modification: Blending modification involves blending or compounding PBT with modifiers or high-impact-strength materials, distributing them as a dispersed phase within the PBT matrix. This utilizes the partial compatibility of the two components or appropriate interfacial bonding to improve the notched impact resistance of PBT. For example, adding the reactive compatibilizer POE-g-GMA to PBT enhances interfacial forces through the in-situ compatibilization reaction between GMA and the terminal carboxyl groups of PBT, achieving a toughening effect.

 

II. PBT Thin-Wall Products Require Higher Flowability

 

In the electronics, electrical appliances, and automotive electronics industries, thinner components are a trend. This necessitates materials with higher flowability to achieve mold filling with minimal filling pressure or clamping force from the corresponding gating equipment. Utilizing low-viscosity thermoplastic polyester compositions often allows for shorter cycle times. Furthermore, good flowability is also crucial for highly filled thermoplastic polyester compositions, such as those containing over 40% glass fiber and/or minerals by mass.

 

Solutions:

 

1. Choose low molecular weight PBT; however, lower molecular weight will affect mechanical properties. 2. Flow promoters such as stearates or lignite esters can improve PBT flowability, but these low-molecular-weight esters can leach out during product processing and use.

 

3. For PBT materials requiring toughening, the addition of toughening agents will inevitably lead to a decrease in flowability; therefore, toughening agents with less impact on flowability should be selected.

 

4. Adding similar low-molecular-weight polyesters with specific structures, such as CBT, can improve flowability. CBT is a functional resin with a macrocyclic oligopolyester structure and has excellent compatibility with PBT. Very small amounts can significantly improve resin flowability with almost no impact on mechanical properties.

 

5. Adding nanomaterials can improve PBT flowability. Ideally dispersed nanomaterials act as an internal lubricant in PBT, but dispersing nanofillers is a major challenge in the blending modification process.

 

III. Glass fiber reinforced PBT materials are prone to warping.

 

Warping is a result of uneven material shrinkage. Warpage in PBT/GF composites can be caused by a variety of factors, including the orientation and crystallization of components in the material, inappropriate injection molding process conditions, incorrect gate shape and location in mold design, and uneven wall thickness in product design.

 

Warpage in PBT/GF composites is primarily due to the orientation of glass fibers in the flow direction restricting resin shrinkage. The induced crystallization of PBT around the glass fibers further enhances this effect, resulting in less longitudinal (flow direction) shrinkage than transverse (perpendicular to the flow direction). This uneven shrinkage leads to warpage in the PBT/GF composite.

Solutions:

 

1. Add minerals to mitigate the anisotropy caused by glass fiber orientation using the symmetry of the mineral filler's shape.

 

2. Add amorphous materials to reduce the crystallinity of PBT and decrease uneven shrinkage caused by crystallization. Examples include ASA or AS. However, these have poor compatibility with PBT, requiring the addition of appropriate compatibilizers.

 

3. Adjust the injection molding process, such as appropriately increasing the mold temperature and extending the injection cycle.

 

IV. Surface Floating Fiber Problem in Glass Fiber Reinforced PBT

 

The causes of floating fiber are complex, but can be summarized as follows:

 

1. Poor compatibility between PBT and glass fiber prevents them from bonding effectively.

 

2. Significant viscosity difference between PBT and glass fiber causes them to tend to separate during flow. When the separation force exceeds the adhesive force, detachment occurs, and the glass fiber floats to the outer layer and is exposed.

 

3. The presence of shear force leads to local viscosity differences and damages the interface layer on the glass fiber surface. The lower the melt viscosity, the more damaged the interface layer, and the weaker the adhesive force on the glass fiber. When the viscosity drops to a certain level, the glass fiber breaks free from the PBT resin matrix and gradually accumulates on the surface, becoming exposed.

 

4. The influence of mold temperature. Due to the low temperature of the mold surface, the lightweight and rapidly cooling glass fiber is instantly frozen. If it cannot be fully surrounded by the melt in time, it will be exposed, forming "floating fiber."

 

Solutions:

 

1. Add compatibilizers, dispersants, and lubricants to improve the floating fiber problem. 1. Using specially surface-treated glass fiber, or adding compatibilizers (such as SOG, a good-flow PBT modifier compatibilizer), can increase the adhesion between PBT and glass fiber through a "bridging" effect.

 

2. Optimizing the molding process to improve fiber floating. Higher injection and mold temperatures, greater injection and back pressures, faster injection speeds, and lower screw speeds can all improve fiber floating to some extent.

 

V. Excessive Mold Scrap During Glass Fiber Reinforced PBT Injection Molding

 

Mold fouling is caused by excessively high small molecule content or poor thermal stability of the material. PBT, with its oligomer and small molecule residual rates typically between 1% and 3%, is relatively prone to mold fouling compared to other materials. This is even more pronounced after the introduction of glass fiber. This necessitates periodic mold cleaning during continuous processing, resulting in low production efficiency.

 

Solutions:

 

1. Reduce the amount of small molecule additives (such as lubricants, coupling agents, etc.) and choose high molecular weight additives whenever possible;

 

2. Improve the thermal stability of PBT and reduce small molecule products generated during thermal degradation in processing.

 

VI. Poor Resistance to Hot Water Hydrolysis in PBT

 

The main factor affecting PBT hydrolysis is the concentration of terminal carboxyl groups. Because PBT contains ester bonds, these bonds break when placed in water at temperatures above its glass transition temperature. The acidic environment created by hydrolysis accelerates the reaction, leading to a sharp decline in performance.

 

Hongrenwei is the preferred choice for injection molds.

 

Solutions:

 

1. Add hydrolysis stabilizers, such as carbodiimide. These stabilizers consume the carboxyl groups generated during hydrolysis, slowing down the acidic hydrolysis rate of PBT and improving the hydrolysis resistance of the PBT resin.

 

2. By blocking the terminal carboxyl groups of PBT, the concentration of terminal carboxyl groups can be reduced, thereby improving the hydrolysis resistance of PBT. For example, adding auxiliaries with epoxy functional groups (such as the SAG series, a random copolymer of styrene-acrylonitrile-GMA) can be used to block the terminal carboxyl groups of PBT through the reaction of the functional group GMA with the terminal carboxyl groups, thereby improving the hydrolysis resistance of PBT.