For polypropylene (PP) materials used in automotive applications, the coefficient of linear expansion (CLTE) is a crucial variable. In general, the CLTE of PP material is rather large, and during injection molding, the CLTE of the melt flow (MD) direction and the vertical flow (TD) direction are significantly different. The MD direction, which cannot satisfy the demand for automobile parts, has a TD direction that is approximately two to three times more. criteria for dimensional stability. It is possible to decrease the mismatch and assembly gap with other assemblies as well as enhance the assembly effect by lowering the CLTE of the PP material and enhancing its dimensional stability.

Describe CLTEs.

The term CLTE stands for the relationship between the starting length (L) and the length that the material changes to over a range of unit temperature change (T); CLTE=L/(LT). The assembly of the material’s pieces and the dimensional stability after assembly are both influenced by the CLTE of the material, which has a strong correlation with the material’s size. Static thermomechanical analysis (TMA) is used to measure CLTE. Using a non-vibrational load and programmable temperature control, a substance’s deformation and temperature are measured using TMA. The measured curve has the sample length L or sample deformation L as the ordinate and temperature T as the abscissa.

Effects of PP resin

The molecular chains of PP, a semi-crystalline polymer with a certain degree of crystallinity, will be directed along the direction of melt flow during the injection molding process. The thermal expansion parameters of PP amorphous segments are greater than those of crystalline segments due to their increased mobility. Therefore, enhancing the PP material’s crystallinity and molecular chain orientation can lower the CLTE of the substance.

Elastomer tougheners’ impact

Although PP materials for automobiles typically have good toughness, PP itself has low toughness, hence elastomer tougheners are required for toughening modification. Ethylene-propylene diene monomer (EPDM), ethylene-alpha olefin elastomer (POE), and styrene-based thermoplastic elastomers are the most often utilized elastomer tougheners.

01Elastomer Morphology Effect

The CLTE of PP composites is significantly influenced by the shape of the elastomer rubber phase. The image below provides a schematic representation of the thermal expansion behavior of various rubber morphologies in plastic/rubber mixtures.

The viscosity ratio of the elastomer/PP matrix has a significant impact on the shape of the elastomer. The elastomer is rod-shaped along the MD and TD directions when the elastomer/PP viscosity ratio is low, and the PP crystallizes in the direction opposite to the elastomer orientation. The elastomer takes on a rod-shaped appearance in the MD direction and a spherical appearance along the TD direction when the viscosity ratio of the elastomer/PP is medium. PP is aligned perpendicular to the elastomer’s orientation in the MD direction. PP randomly pierces the elastomer in the TD direction. The crystal is oriented downward.

When the elastomer/PP viscosity ratio is high, PP penetrates the elastomer randomly, the crystallographic orientation further declines, and the elastomer becomes circular in both the MD and TD directions, indicating that it is spherical. The CLTE increases in the MD and TD directions and decreases in the thickness direction when the elastomer/PP viscosity ratio rises.

02Elastomer Content’s Effect

The composition of the elastomer, in addition to its shape, significantly affects the CLTE of the PP material.

When the EPR component is lower than 20%, the CLTE of the PP/ethylene-propylene rubber (EPR) alloy progressively increases, and then it sharply declines. as a result, the CLTE of PP/EPR alloy rises quickly. The CLTE of PP/EPR alloy is 4.310-5°C-1 when the elastomer EPR content is 60%, which is lower than the CLTE of 30% talc filling (5.010-5°C-1) and equivalent to that of 30% Glass fiber filling (3.510-5°C-1). As a result, the material’s CLTE may be decreased without having a lot of filler, allowing it to be used in low density products.

Ethylene and co-polymers are combined to create POE. The amount and kind of POE’s constituents will have an impact on the CLTE of PP materials. Phase separation happens when PP and elastomers are combined. Phase separation occurs slowly when the comonomer content is high and quickly when the comonomer concentration is low.

The phase separation of PP amorphous chains from the elastomer is sluggish for POE with a high comonomer content (30%), which is prevented by the quick crystallization of PP, leaving the PP amorphous chains in the elastomer. As a result, crystalline PP reduces the thermal expansion of PP amorphous segments and elastomers, reducing the CLTE. The PP amorphous chain will diffuse into the PP crystal in POE with a low comonomer content (9%), and since the thermal expansion of the elastomer and the PP amorphous segment is less constrained, CLTE will be bigger. when a result, when the comonomer content of POE elastomers rises, so does the CLTE of PP materials.

Additionally, the CLTE of PP is significantly impacted by the melt fluidity of POE. During the shear dispersion process of blended materials, POE is more easily dispersed and forms a continuously distributed microscopic phase state when the melt flow rate (MFR) of POE increases. The dispersed rubber phase is bound by PP in the structure, which suppresses the thermal expansion behavior of the PP material in the MD and TD directions. As a result, the PP material’s CLTE gradually decreases.

results of filler

Inorganic fillers are frequently employed for reinforcement modification in PP materials to increase stiffness. A few of the most often used reinforcing fillers are flaky talcum powder, mica, needle-shaped whiskers, wollastonite, and glass fibers with a certain length-to-diameter ratio. Another common filler is spherical calcium carbonate, of which talc powder is the most common. The CLTE of polymer materials can be greatly decreased by adding these inorganic fillers because of their extremely low CLTE.

Among these, talc powder particle size reduction and talc powder concentration increases may both decrease the CLTE of PP in the MD and TD directions.

Maleic anhydride grafted PP (PP-g-MAH) can be used as a compatibilizer to make the polymer chains entangle with one another, interpenetrate, and mix with the filler at the same time, improving the interfacial binding force between the filler and PP. The CLTE of the PP material is decreased due to the suppression of thermal movement, and the decrease is larger in the direction perpendicular to the flow.

Effects of additives

Nucleating agents are frequently applied to modified PP materials to enhance the material’s crystallinity and stiffness. Polymer chains are less free to move and experience less thermal expansion in crystalline areas than they do in amorphous ones. As a result, the CLTE of the PP material without the nucleating agent is lower than that of the PP material with the nucleating agent.

A lot of factors affect CLTE, including PP resin, elastomer, filler, and nucleating agent. The following conclusions can be reached by summarizing the approaches mentioned above:

CLTE can be decreased by enhancing the crystallinity and molecular chain orientation of PP resin.

Reduced CLTE can be achieved by altering the morphology of the elastomer to form a bicontinuous phase structure with the PP matrix, increasing the comonomer content of the elastomer, lowering the degree of phase separation, raising the elastomer’s MFR, reducing its particle size, and increasing its MFR.

CLTE can be greatly decreased by adding inorganic fillers with various structural variations.

The crystallinity of PP can be increased while lowering CLTE by adding a nucleating agent.