Polybutanediol terephthalate (PBT) has excellent comprehensive properties, such as high crystallinity, rapid prototyping, weather resistance, low friction coefficient, high thermal deformation temperature, good electrical properties, excellent mechanical properties, fatigue resistance, can be ultrasonic welding.

However, its notched impact strength is low, forming shrinkage rate is large, hydrolysis resistance is poor, easy to be eroded by halogenated hydrocarbons, after glass fiber reinforcement, because of the product longitudinal and horizontal shrinkage is inconsistent easy to warping products.

Below, let's look at the causes and solutions of common problems in PBT modification.

Notch sensitivity

(1) Reasons
The benzene ring and ester group in PBT molecule form a large conjugate system, which reduces the flexibility of the molecular chain and increases the molecular rigidity. Moreover, the presence of polar ester group and carbonyl group increases the intermolecular force and further enhances the molecular rigidity, resulting in poor toughness.

(2) Solutions
a) polymerization modification
Polymerization modification is to introduce new flexible chain segments into PBT molecules in the polymerization process by means of copolymerization, grafting, block, crosslinking and so on, so as to make it have good toughness.

b) blending modification
Blending modification is to blend or compound the modifier or high impact strength material with PBT, so that it is distributed in the PBT matrix as a dispersed phase, using the partial compatibility of the two components or appropriate interface bonding to improve the notch impact performance of PBT. For example, POE-g-GMA, a reactivity compatibilist, is added to PBT to enhance the interface force through in-situ compatibilization reaction between GMA and the terminal carboxyl group of PBT, so as to achieve toughening effect.

PBT thin-walled products require higher mobility

(1) Reasons

In the field of electronics, automotive and electronic industries, thinner components are the trend, which requires higher mobility of materials, in order to achieve mold filling with as little as possible the corresponding pouring instrument filling pressure or mold closing force. Shorter cycles are also often achieved using thermoplastic polyester compositions with low viscosity. In addition, good flow ability is important for example for highly packed thermoplastic polyester compositions with a mass fraction of more than 40% glass fiber and/or minerals.

(2) Solutions

a) Select low molecular weight PBT, but the reduction of molecular weight will affect the mechanical properties;
b) PBT liquidity can be improved with flow accelerators such as stearate or lignite ester, but these low molecular weight esters will leak out during product processing and use;
c) For the PBT material that needs toughening, the addition of the toughening agent will certainly lead to the decrease of liquidity, so it is necessary to choose the toughening agent that has less influence on the liquidity;
d) Adding similar low molecular polyester with specific structure, such as CBT, CBT is a functional resin with macrocyclic oligopolypet structure, and has good compatibility with PBT, very little addition amount, can greatly improve the fluidity of the resin, and almost does not affect the mechanical properties;
e) When nanomaterials are added, the ideally dispersed nanomaterials play a role similar to internal lubrication in PBT, which can improve the fluidity of PBT. However, the dispersion of nano fillers is a major difficulty in the blending modification process.

Glass fiber reinforced PBT material warps easily

(1) Reasons
Warping is the result of uneven shrinkage of the material. The warping of the product can be caused by the orientation and crystallization of the components in the material, the improper technological conditions used in the injection molding, the wrong shape and position of the gate in the mold design, and the uneven thickness of the wall in the product design.

The warping of PBT/GF composites is mainly due to the fact that the orientation of the glass fiber in the flow direction restricts the shrinkage of the resin, and the induced crystallization of PBT around the glass fiber strengthens this effect, making the longitudinal (flow direction) shrinkage of the product less than the transverse (perpendicular to the flow direction). This uneven shrinkage leads to the warping of PBT/GF composites.

(2) Solutions
a) Add minerals. The shape symmetry of mineral fillers is used to reduce the anisotropy caused by glass fiber orientation.
b) Add amorphous materials. Reduce the crystallinity of PBT, reduce the uneven shrinkage caused by crystallization, such AS adding ASA or AS, but they have poor compatibility with PBT, need to add appropriate compatibilizers;
c) Adjust the injection molding process. If appropriate increase mold temperature, appropriate increase injection cycle.

Glass fiber reinforced PBT surface floating fiber problem

(1) Reasons

The causes of floating fiber are more complex. In short, there are mainly the following aspects:

(1) The compatibility of PBT and glass fiber is very poor, resulting in the two can not effectively bond together;
(2) The viscosity of PBT and glass fiber is very different, resulting in a tendency of separation between the two in the flow process. When the separation effect is greater than the adhesive force, the separation will occur, and the glass fiber will float to the outer layer and leak out.
(3) The presence of shear force will not only lead to differences in local viscosity, but also destroy the interfacial layer melt viscosity on the glass fiber surface. The smaller the melt viscosity, the damaged interfacial layer, the smaller the bonding force on the glass fiber. When the viscosity reaches a certain level, the glass fiber will get rid of the binding of PBT resin matrix and gradually accumulate to the surface and expose.
(4) Influence of mold temperature. Due to the low temperature of the mold surface, the glass fiber with light weight and fast condensation is frozen instantaneously. If it is not fully surrounded by melt in time, it will be exposed and form "floating fiber".

(2) Solutions
Compatibilizers, dispersants and lubricants were added to improve the floating fiber problem. Such as the use of special surface treatment of glass fiber, or add compatibilizers (such as: SOG, a good flow PBT modified compatibilizer), through the role of "bridge", increase the adhesion of PBT and glass fiber.
Optimization of forming process to improve floating fiber problem. Higher injection molding temperature and mold temperature, larger injection molding pressure and back pressure, faster injection molding speed, lower screw speed, can improve the floating fiber problem to a certain extent.

The glass fiber reinforced PBT injection process produces more mold scale

(1) Reasons
The formation of mold scale is caused by the high content of small molecules or the poor thermal stability of materials. Compared with other materials, PBT is easy to generate mold scale due to its oligomer and small molecule residue rate usually in the range of 1%-3%. And after the introduction of glass fiber, more obvious. This will lead to the continuous processing process, the need to clean the mold regularly, resulting in low production efficiency.

(2) Solutions
Reduce the amount of small molecular additives (such as lubricant, coupling agent, etc.), try to choose polymer additives;
Improve the thermal stability of PBT, reduce the thermal degradation of small molecular products during processing;

The thermal hydrolysis of PBT is poor

(1) Reasons
The main factor affecting the hydrolysis of PBT is the concentration of terminal carboxyl group. Because PBT contains ester bonds, the ester bonds will break when placed in water at a temperature higher than the glass transition temperature. The acidic environment formed by hydrolysis accelerates the hydrolysis reaction and sharply decreases the performance.

(2) Solutions
Adding a hydrolysis stabilizer, such as carbonized diimide, will consume the carboxyl group produced by hydrolysis, slow down the acid hydrolysis rate of PBT, and improve the hydrolysis resistance of PBT resin.
By blocking the terminal carboxyl group of PBT, the concentration of terminal carboxyl group can be reduced to improve the hydrolysis resistance of PBT. For example, additives with epoxy functional group (such as SAG series, a random copolymer of styrene - acrylonitrile -GMA) are added, and the terminal is sealed by the reaction of the functional group GMA with the terminal carboxyl group of PBT, so as to improve the hydrolysis resistance of PBT.