Polybutylene terephthalate (PBT) has excellent comprehensive properties, such as high crystallinity, fast moldability, weather resistance, low friction coefficient, high heat - distortion temperature, good electrical properties, excellent mechanical properties, fatigue resistance, and suitability for ultrasonic welding. However, it has low notched impact strength, large molding shrinkage rate, poor hydrolysis resistance, and is vulnerable to halogenated hydrocarbon erosion. After being reinforced with glass fiber, the product is prone to warping due to inconsistent longitudinal and transverse shrinkage rates.
In PBT molecules, the benzene ring and ester group form a large conjugated system, reducing the flexibility of the molecular chain and increasing molecular rigidity. The presence of polar ester and carbonyl groups further enhances intermolecular forces and molecular rigidity, resulting in poor toughness.
Solutions:
Polymer Modification: Introduce new flexible segments into PBT molecules during polymerization through methods like copolymerization, grafting, block - copolymerization, and cross - linking to improve toughness.
Blending Modification: Blend or compound modifiers or high - impact - strength materials with PBT. They are distributed as dispersed phases in the PBT matrix. Utilize partial compatibility or appropriate interfacial adhesion between the two components to enhance the notched impact performance of PBT. For example, add reactive compatibilizer POE - g - GMA to PBT. In - situ compatibilization reaction between GMA and the terminal carboxyl groups of PBT strengthens interfacial forces for a toughening effect.
In the fields of electronics, electrical appliances, and automotive electronics, thinner components are the trend. This requires materials with higher fluidity to fill the mold with the lowest possible filling pressure or clamping force of the corresponding pouring equipment. Using low - viscosity thermoplastic polyester compositions can often achieve shorter cycle times. Good flowability is also crucial for highly filled thermoplastic polyester compositions with glass fibers and/or minerals with a mass fraction exceeding 40%.
Solutions:
Select low - molecular - weight PBT, but a decrease in molecular mass will affect mechanical properties.
Use flow promoters such as stearates or montanates to improve PBT fluidity, but these low - molecular - mass esters may exude during product processing and use.
For PBT materials that need toughening, the addition of toughening agents will inevitably reduce fluidity. So, choose toughening agents that have less impact on fluidity.
Add low - molecular - weight polyesters of the same kind with a specific structure, such as CBT. CBT is a functional resin with a macrocyclic oligomeric polyester structure, highly compatible with PBT. A small addition amount can greatly increase the resin's fluidity with little impact on mechanical properties.
Add nanomaterials. Ideally dispersed nanomaterials act like internal lubricants in PBT to improve fluidity, but dispersing nanofilers is a major challenge in the blending modification process.
Warping is the result of non - uniform shrinkage of materials. Factors such as component orientation and crystallization in the material, inappropriate processing conditions during injection molding, incorrect gate shape and position in mold design, and uneven wall thickness in product design can all cause product warping.
For PBT/GF composites, warping mainly occurs because the orientation of glass fibers in the flow direction restricts resin shrinkage, and the induced crystallization of PBT around the glass fibers reinforces this effect. As a result, the longitudinal (flow direction) shrinkage of the product is less than the transverse (perpendicular to the flow direction) shrinkage. This non - uniform shrinkage leads to the warping of PBT/GF composites.
Solutions:
Add minerals. The shape symmetry of mineral fillers can reduce the anisotropy caused by glass fiber orientation.
Add amorphous materials to reduce the crystallinity of PBT and decrease non - uniform shrinkage caused by crystallization, such as adding ASA or AS. However, they have poor compatibility with PBT, so appropriate compatibilizers are needed.
Adjust the injection molding process, such as appropriately increasing the mold temperature and injection cycle.
The causes of fiber blooming are complex. Mainly:
Poor compatibility between PBT and glass fibers prevents effective bonding.
Large viscosity differences between PBT and glass fibers lead to a separation tendency during flow. When the separation force is greater than the adhesive force, detachment occurs, and glass fibers float to the outer layer and are exposed.
Shear force causes local viscosity differences and damages the interfacial layer on the surface of glass fibers. The smaller the melt viscosity, the more damaged the interfacial layer, and the smaller the adhesive force on the glass fibers. When the viscosity is small enough, the glass fibers break free from the PBT resin matrix and gradually accumulate on the surface and become exposed.
Mold temperature impact. Since the mold surface temperature is low, light - weight glass fibers with fast condensation are instantaneously frozen. If they are not fully surrounded by the melt in time, they will be exposed and form "fiber blooming".
Solutions:
Add compatibilizers, dispersants, and lubricants to improve the fiber blooming problem. For example, use glass fibers with special surface treatment or add compatibilizers (such as SOG, a good - flow PBT modification compatibilizer) to increase the adhesive force between PBT and glass fibers through a "bridge" effect.
Optimize the molding process to improve the fiber blooming problem. Higher injection and mold temperatures, greater injection pressure and back pressure, faster injection speed, and lower screw speed can all improve the problem to some extent.
Mold deposits are caused by a high content of small molecules in the material or poor thermal stability. PBT is relatively prone to mold deposits because its oligomer and small - molecule residue rate is usually 1% - 3%. This is more obvious after the introduction of glass fibers. This requires regular mold cleaning during continuous processing, resulting in low production efficiency.
Solutions:
Reduce the addition amount of small - molecule additives (such as lubricants and coupling agents) and preferentially choose high - molecular - weight additives.
Improve the thermal stability of PBT to reduce small - molecule products generated by thermal degradation during processing.
The main factor affecting PBT hydrolysis is the terminal carboxyl group concentration. Since PBT contains ester bonds, the ester bonds will break when placed in water at a temperature above its glass transition temperature. The acidic environment formed by hydrolysis accelerates the hydrolysis reaction, causing a sharp decline in performance.
Solutions:
Add hydrolysis stabilizers, such as carbodiimides. Hydrolysis stabilizers consume the carboxyl groups generated by hydrolysis, slowing down the acidic hydrolysis rate of PBT and improving its hydrolytic resistance.
Reduce the terminal carboxyl group concentration by blocking the terminal carboxyl groups of PBT to improve its hydrolytic resistance. For example, add additives with epoxy functional groups (such as the SAG series, a random copolymer of styrene - acrylonitrile - GMA). React the functional group GMA with the terminal carboxyl groups of PBT for end - capping, thereby improving the hydrolytic resistance of PBT.
