Polybutylene terephthalate has excellent comprehensive properties, such as high crystallinity, rapid molding, weather resistance, low friction coefficient, high heat distortion temperature, good electrical properties, excellent mechanical properties, fatigue resistance, ultrasonic Welding, etc. However, the notched impact strength is low, the molding shrinkage is large, the hydrolysis resistance is poor, and it is vulnerable to attack by halogenated hydrocarbons. After being reinforced by glass fiber, warpage may occur in the product due to inconsistent longitudinal and transverse shrinkage of the product.
Below, briefly explain the causes and solutions of common problems in PBT modification.
one, Notch sensitivity
the reason:
The benzene ring and the ester group in the PBT molecule form a large conjugated system, which reduces the flexibility of the molecular chain, increases the rigidity of the molecule, and increases the intermolecular force due to the presence of polar ester groups and carbonyl groups. Further enhanced, and therefore poor toughness.
Solution:
a) Polymerization modification
Polymerization modification is the introduction of new flexible segments in the PBT molecules through copolymerization, grafting, block, cross-linking and other means to make it have good toughness.
b) Blend modification
The blending modification is blending or compounding the modifier or high impact strength material with PBT to distribute it as a dispersed phase in the PBT matrix, utilizing the partial compatibility of the two components or appropriate interfacial bonding to increase the PBT. The notch impact properties. If the reactive compatibilizer POE-g-GMA is added in PBT, the in-situ compatibilization of the carboxyl-terminated carboxyl groups of GMA and PBT will strengthen the interfacial forces to achieve the toughening effect.
two, PBT thin-walled products require higher fluidity
Thin-walled electronic components
In the electronics and automotive, automotive electronics industry, the trend towards thinner components is that this requires a higher fluidity of the material in order to fill the mold with the filling pressure or mold clamping force of the smallest possible casting device. Low-viscosity thermoplastic polyester compositions also often achieve shorter cycle times. In addition, good flowability is also for high-filled thermoplastic polyester compositions such as glass fibers and/or minerals having a mass fraction of more than 40%. very important.
Solution:
Low-molecular-weight PBT is selected, but the reduction in molecular weight will affect the mechanical properties.
With the aid of flow promoters such as stearates or montanates, PBT fluidity can be improved, but such low molecular weight esters can leach out during processing and use of the product.
For PBT materials that need to be toughened, the addition of tougheners will definitely lead to a drop in fluidity, so it is necessary to choose tougheners that have less impact on fluidity.
Add the same kind of low-molecular polyester with a specific structure, such as CBT, CBT is a functional resin with a macrocyclic oligo-polyester structure, with PBT has a good compatibility, a very small amount of addition, you can significantly Improves the fluidity of the resin without affecting the mechanical properties.
Adding nanomaterials, ideally dispersed nanomaterials play a role similar to internal lubrication in PBT, which can improve the flowability of PBT, but the dispersion of nanofillers is a major difficulty in the process of blend modification.
three, Glass fiber reinforced PBT material is prone to warping
the reason:
Warpage is the result of uneven shrinkage of the material. The orientation and crystallization of the components in the material, improper process conditions during injection molding, improper shape and location of the gate during mold design, uneven thickness and thickness of the product during design, etc. Warping.
The warpage of PBT/GF composites is mainly due to the fact that the orientation of the glass fiber in the flow direction limits the shrinkage of the resin. The induced crystallisation of PBT around the glass fiber reinforces this effect, making the longitudinal (flow direction) shrinkage of the article smaller than the transverse direction. (in a direction perpendicular to the direction of flow), this uneven shrinkage leads to warping of the PBT/GF composite.
Solution:
One is the addition of minerals, which uses the shape symmetry of mineral fillers to reduce the anisotropy caused by the orientation of the glass fibers;
The second is to add amorphous materials to reduce the crystallinity of PBT and reduce the non-uniform shrinkage caused by crystallization, such as adding ASA or AS, but their compatibility with PBT is poor, and it is necessary to add an appropriate compatibilizer;
The third is to adjust the injection molding process, such as an appropriate increase in mold temperature, an appropriate increase in the injection cycle.
four, Glass fiber reinforced PBT surface fiber problem
the reason:
The causes of floating fibers are more complex. Simply put, there are mainly the following aspects:
The poor compatibility of PBT and glass fiber results in ineffective bonding between the two.
The viscosity of PBT and glass fiber is very different, which leads to a tendency of separation between the two in the flow process. When the separation effect is greater than the adhesive force, detachment occurs, and the glass fiber floats to the outer layer and leaks out.
The presence of shear force will not only cause local viscosity difference, but also will destroy the interfacial layer on the surface of glass fiber. The lower the melt viscosity is, the interface layer will be damaged, and the glass fiber will have less adhesion, when the viscosity is small to some extent. At that time, the glass fiber will get rid of the bond of the PBT resin matrix and gradually accumulate on the surface and expose.
Impact of mold temperature. Due to the low mold surface temperature, the glass fiber with light weight and rapid cooling is instantly frozen. If it is not fully surrounded by the melt in time, it will be exposed to form a 'float fiber'.
Solution:
Adding compatibilizers, dispersants and lubricants to improve floatation problems. If using special surface treated glass fiber, or adding compatibilizer (eg: SOG, a good flow PBT modified compatibilizer), pass the 'bridge' The role of PBT and glass fiber adhesion.
Optimize the molding process to improve the floating fiber problem. Higher injection temperature and mold temperature, larger injection pressure and back pressure, faster injection speed, lower screw speed, can improve the floating fiber problem to some extent.
Five Glass-reinforced PBT Injection Processes Produce More Mold Scaling
PBT+GF surface floc, mould scale phenomenon
the reason:
Mold scale generation is caused by too high a small molecule content of the material or poor thermal stability of the material. PBT is often 1% to 3% due to its oligomers and small molecule residuals, and is prone to mold deposits with other materials. And after the introduction of glass fiber, it is more obvious. This will lead to the need to regularly clean the mold in the continuous processing process, resulting in low production efficiency.
Solution:
Reduce the amount of small molecule additives (such as lubricants, coupling agents, etc.)
Improve the thermal stability of PBT and reduce the production of small molecules by thermal degradation during processing;
Fives, PBT poor heat hydrolysis
the reason:
The main factor influencing the hydrolysis of PBT is the carboxyl-terminated concentration. Since PBT contains ester bonds, ester bond cleavage occurs when placed in water at a temperature above its glass transition temperature, and the hydrolysis creates an acidic environment that accelerates the hydrolysis reaction, leading to a dramatic drop in performance. .
Solution:
Adding hydrolysis stabilizers, such as carbodiimides, the hydrolysis stabilizer will consume the carboxyl groups produced by the hydrolysis, slowing down the acid hydrolysis rate of PBT, and improving the hydrolysis resistance of PBT resin.
By blocking the carboxyl group of PBT, the concentration of carboxyl groups is lowered, and the hydrolysis resistance of PBT is improved. For example, additives with epoxy functional groups (such as the SAG series, a styrene-acrylonitrile-GMA random copolymer) can be added. Through the functional group GMA and PBT terminal carboxyl end capping, thereby enhancing the hydrolysis resistance of PBT.