Factors Affecting The Shrinkage Of Rubber Compounds

1. Introduction

1.1 Importance of Shrinkage Rate of Rubber Products

One crucial performance metric for rubber goods is their shrinkage rate. It has a direct impact on the items' performance, appearance, and dimensional correctness. A high rate of shrinkage may result in surface flaws, product size variation, and other issues that will negatively impact the quality of the product. Consequently, one of the most important technologies in the rubber manufacturing process is managing and maximizing the shrinkage rate of rubber goods.

1.2 Overview of the main factors affecting the shrinkage rate

• Rubber formula: The shrinkage rate will vary depending on the kinds and amounts of fillers, plasticizers, and raw materials used in the rubber.
• Molding procedure: Shrinkage behaviors will vary depending on the kind of molding used, such as injection molding or compression molding.
• Molding conditions: The shrinkage rate is significantly impacted by the choice of process parameters, including temperature, pressure, and holding time.
• Post-processing: The ultimate shrinkage rate will also be impacted by later process steps such demolding, cooling, and heat treatment.

2. The influence of formula factors on shrinkage rate

2.1 The influence of different rubber types on shrinkage rate

2.1.1 Natural rubber, styrene-butadiene rubber, chloroprene rubber, etc.

One of the key elements influencing shrinkage rate is rubber type. The shrinkage behaviors of various rubber kinds vary because of variations in their molecular structures, polarity, crosslinking densities, and other properties.

• Because natural rubber (NR) has a longer molecular chain and a lower crosslinking density-usually between 10 and 15 percent-it shrinks more quickly.
• Styrene monomer is added to styrene-butadiene rubber (SBR), which results in increased molecular polarity, a greater crosslinking density, and a decreased shrinkage rate (usually 5–10%) compared to natural rubber.
• Chloroprene rubber (CR) is characterized by the presence of chlorine atoms, a greater crosslinking density, a stronger molecular polarity, and a lower shrinkage rate (often 3-8%).
• Others, including nitrile rubber (NBR) and ethylene-propylene rubber (EPDM), have distinct shrinkage properties because of their varied molecular polarities and structures, which call for more investigation.

2.2 Effect of filler content on shrinkage

2.2.1 Inorganic fillers vs. organic fillers

Rubber product shrinkage rate is also significantly influenced by the kind and quantity of fillers used. Typically, they are separated into two groups: inorganic and organic fillers.

• The most common types of inorganic fillers include talc, dolomite powder, white carbon black, and carbon black. Because they are more stiff and have a higher modulus, this kind of filler may typically restrict the amount that the rubber matrix shrinks.
• Since organic fillers like cellulose and wood powder shrink more than other fillers do, adding too much of them can cause the total shrinkage to rise.

2.2.2 Relationship between filler content and shrinkage

Generally speaking, the rubber product will shrink less the more filler there is. This is due to the fact that stiff fillers may prevent the rubber matrix from deforming due to shrinking.

On the other hand, an excessive filler content would degrade the product's mechanical qualities and formability. As a result, it's critical to optimize and balance shrinkage control with other performance needs.

Controlling the filler content between 30 and 50 percent is often a wise decision since it may successfully minimize shrinkage without unduly influencing other performance parameters. It is necessary to filter and modify the filler type and content in accordance with the particular needs of the product.

2.3 The influence of other additives

2.3.1 softener, stabilizer, colorant, etc.

Softener:

Rubber can elongate and become more plastic with plasticizer, but shrinkage will also increase. In general, shrinkage increases with increasing plasticizer concentration.

Stabilizer：

By raising the crosslinking density of rubber, stabilizers such certain antioxidants and antiozonants may be added to decrease shrinkage. Nevertheless, embrittlement can also result from overadding.

Additive:

Unless the addition amount is very high, the introduction of colorants like pigment and dye typically has no discernible influence on shrinkage.

Others:

A little quantity of vulcanizing agent, accelerator of vulcanization, foaming agent, etc., may also have some impact on shrinkage.

3. The influence of process factors on shrinkage

3.1 The influence of mixing conditions on shrinkage

3.1.1 Speed, temperature, time, etc.

Speed of mixing:

A speed that is too fast will break the rubber molecular chain, lowering the molecular weight and increasing shrinkage. Controlling it between 30 and 60 rpm is often preferable.

Temperature of mixing:

Any too high temperature can hasten the rubber molecules' thermal deterioration and cause further shrinking. The mixing temperature is typically maintained between 110 and 160 °C.

Time spent mixing:

An excessively lengthy mixing period can cause shrinkage and additional molecular weight reduction. However, too little time prevents fillers from dispersing completely. Controlling it for three to ten minutes is often adequate.

Order of mixing:

To minimize shrinkage and prevent filler agglomeration, it is advisable to apply hard fillers first, followed by soft rubber.

3.2 Effect of vulcanization conditions on shrinkage

3.2.2 Vulcanization temperature, time, pressure, etc.

Temperature of vulcanization:

An excessively high temperature can hasten the rubber chains' cross-linking and thermal deterioration, which will increase shrinkage. It is regularly preferable to keep it between 150 and 180 °C.

Time of vulcanization:

An excessive amount of time can accelerate shrinking and encourage the cross-linking reaction. However, vulcanization is not fully achieved in too short a period. Usually within 10 to 30 minutes.

Pressure during vulcanization:

The rubber's volume will be compressed by excessive pressure, which will accelerate shrinking. Maintaining pressure management between 5 and 15 MPa is appropriate.

3.3 Effect of molding process on shrinkage

3.3.1 Injection molding vs extrusion

Rapid injection and cooling procedures in injection molding will cause more shrinkage. Extrusion molding shrinks less quickly and is comparatively sluggish.

3.3.2 Cooling rate

The rate of shrinking increases with cooling speed. Controlling the cooling rate is therefore essential to prevent overcooling.

4. Effect of environmental factors on shrinkage

4.1 Effect of temperature on shrinkage

Rubber's thermal expansion coefficient will rise with temperature, speeding up the rate of shrinkage.

Rubber molecular chains will also experience cross-linking and thermal breakdown at high temperatures, which will exacerbate shrinking.

Rubbers with varying compositions are also temperature-sensitive. Certain types of rubber are more susceptible to variations in temperature.

In order to minimize shrinkage, high temperature settings should be avoided wherever feasible for using and storing rubber products. Passive or active temperature management techniques can be used for important components.

4.2 Effect of humidity on shrinkage

Generally speaking, humidity has little impact on rubber shrinkage. On the other hand, moisture absorption will cause some rubber goods containing absorbent fillers to expand, increasing the overall volume and decreasing shrinkage.

Rubber easily absorbs moisture and softens in high humidity environments, which can lead to unstable product sizes. Overdrying can also have an impact on shrinkage and result in rubber embrittlement.

Thus, it is significant to maintain the proper humidity environment, typically between 40 and 70 percent relative humidity, when utilizing and storing rubber products. When needed, moisture-proof measures can be used.

5. Comprehensive optimization and control

5.1 Optimization of formulation factors

1. Choose rubber raw materials with little shrinking, such silicone or nitrile rubber.
2. To lessen shrinking, decide and combine different fillers-such as talc, white carbon black, and carbon black-reasonably.
3. Add certain shrinkage-controlling substances, such polyethylene, low-molecular-weight polypropylene, etc.
4. Improve the cross-linking system by adding useful chemicals, modifying the sulfur content, etc.

5.2 Optimization of process parameters

• Regulate the pace, timing, and temperature of mixing to prevent over-degradation.
• To regulate the level of crosslinking, adjust the vulcanization temperature, time, and pressure parameters.
• Decide on an appropriate molding technique, such extrusion or injection molding, and regulate the cooling rate.
• Real-time process parameter monitoring and adjustment should be done in conjunction with online detection and other techniques.

5.3 Control of environmental conditions

1. Steer clear of hot environments and, if needed, take appropriate temperature control measures.
2. A modest range of humidity should be maintained to avoid over-drying or moisture absorption.
3. Auxiliary measures like moisture resistance and heat insulation can be used for important components.