Parts per hundred rubber

The basic compounding formulation specifies the minimum requirement of fillers, vulcanizing agents, and other substances that must be added to the rubber compound to achieve the desired properties. After the rubber, cure system and reinforcing filers have been selected it will be necessary to make several adjustments before all requirements are satisfied. It is generally sensible to start with the simplest mix formula for meeting the requirements. The recipe or the formula is usually written on the basis of hundred parts of rubber. For example if 5 parts of zinc oxide is to be added it is denoted as 5 phr (five parts per hundred rubber). Elementary compounding... 

The percentage sulphur (determined in the un-vulcanized reclaim) and the available rubber hydrocarbon are taken into consideration while evolving the compound formulation. It should be noted that reclaimed rubber is not all rubber. In arriving at the total rubber content in the rubber formulation containing reclaim, allowance must be made for its rubber content. For example in the following blend of smoked sheet and whole tyre reclaim, the total rubber content is to be considered as 100 parts instead of 125 parts as below and the proportion of other ingredients should be worked out accordingly on "parts per hundred rubber" (phr) basis. 

In general and as a rule-of-thumb, accelerators of group 1 are used in proportions ranging from 0.5 to 1.5 parts per hundred rubber with sulphur dosages of from 2 to 3 phr. The more active accelerators of groups 2 and 3 should used in proportions from 0.25 to 0.5 phr in which case the sulphur dosage has to be 1.5 to 2 phr. 

Reprinted from [75] with permission of John Wiley and Sons, Inc., 1998 phr parts per hundred rubber... 

Commonly available peroxides that are superficially suitable for rubber and plastic vulcanization can be divided into three classes diacyl peroxides, dialkyl peroxides, and peresters. With diacyl peroxides, only low cross-linking efficiencies are obtained, up to 10 phr (parts per hundred rubber) being required for adequate vulcanization. 

Recent irradiation studies with blends of PVC and modifiers such as flexible polymers (EVA [205] or ENR -epox-idized natural rubber [206]) or PFMs (polyfunctional monomers) have shown that the irradiation achieves more crosslinking and less degradation (chlorine loss) at lower doses. Seven PFMs, used at 10 parts per hundred rubber (phr), were compared for effectiveness for increasing softening temperature, gel yield and swelling ratio in PVC wire formulations [207]. 

Table 1.4 Rubber compound formulas (parts per hundred rubber, phr) ...

Neither natural nor synthetic rubbers can be used as they are produced. They must be mixed with other chemicals to get a balance of properties to suit each end-use. In order to determine the exact proportions in phr (parts per hundred rubber) that are needed for a given end use. 

As stated previously, cobalt salts can be used with or without a resin/silica system and for the latter a combination of all three is required to obtain the desired adhesion and rubber physical properties under all ageing conditions. The concentration of resin is variable but it generally falls in the range of one to four parts per hundred rubber and the methylene donor level is approximately in the same range. For optimum compound properties, it is usual for the resin/donor ratio to be greater than one. Cobalt metal levels generally fall in the range 0.1 to 0.3 parts, whilst silica may used at levels from 0 to 55 parts. 

Kaolin clay is basically used as an economic diluent to reduce the cost of some rubber formulations. If used at all, kaolin is typically used at 20 to 150 parts per hundred rubber. Clay is not used very much in tire applications. However, it is a common raw material for many nontire applications. 

Approximately 1.5 billion worth of curatives were consumed in 2012 by the rubber industry worldwide. Typically, curative ingredients are used at small concentrations of from 0.2 to 5 parts per hundred rubber polymer (phr). 

Generally, products made of rubber require antioxidants to lessen oxidative degradation and extend the product s useful life. Just about every rubber compound contains one to five parts per hundred rubber (phr) of an antioxidant. Over 1 billion worth of antioxidants and antiozonants are used annually by the rubber industry worldwide. 

DNPD is a member of the p-phenylenediamine family of AOs however, it is used more as an effective antioxidant and copper inhibitor (to prevent copper from degrading the cured rubber compound) than it is as an antiozonant (which is how most p-phenylenediamines are used). One to two parts per hundred rubber (phr) of DNPD will impart good heat and oxidative aging resistance with many different types of rubber. DNPD is relatively low staining compared to many other amine AOs. But DNPD can cause some discoloration with bright light exposure. 

Figure 2. Moduli at various elongations (a) and complex storage moduli (b) vs. temperature for NR compounds with and without different reinforcements. Silicate content 10 parts per hundred rubber (phr).

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