Compressive Set Permanent Deformation

The equipment described in the following section is supplied by ATS FAAR (Table 2.6). 

Tests can be carried out under constant load Method A according to ASTM D395-03 [53] or under constant deflection according to ASTM D395 [58], ISO 815 [54], or UNI 6121 [55]. 

FIGURE 2.15 Influence of temperature on Izod impact strength of various polymers of low and high melt indexes. GMT 61 is a low-melt-index and low-notch-sensitivity polypropylene copolymer. KMT 61 is a high-melt-index and low-notch-sensitivity polypropylene polymer. HDPE 65045 is a high-melt-index and high-notch-sensitivity high-density polyethylene. 

The test is used to determine the capability of rubber compounds and elastomers in general to maintain their elastic properties after prolonged action of compression stresses. According to Method A of ASTM D395 [53], this stress is applied by a constant load. 

---

The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

Elastomers are often compounded with finely divided solids to reinforce the rubber and to reduce costs. The most important fillers are carbon blacks, silica and silicates, clays and whiting (calcium carbonate). " The particles are the source of reinforcement through their interactions with the rubber, among themselves and with the chemistry of the cross-linking process. Abrasion resistance, tear strength and tensile strength are simultaneously improved. However, hysteresis, heat build-up and compression set (permanent deformation) are also known to increase as the reinforcing ability of the filler becomes more pronounced. 

The preceding research on the model maxillofacial material was followed by TMDSC study of several representative elastomeric impression materials, which are extensively used in dentistry for the accurate fabrication of inlays and crowns from dental alloys, metal-ceramic restorations, and fixed and removable partial dentures [1-3]. There have been numerous studies reporting the clinically relevant properties of these impression materials (viscosity before setting by polymerization, strain in compression after setting, permanent deformation for simulated in vivo removal of the impressions, and tear strength of the thin impressions). However, only minimal research has been reported [44] on some thermal properties of impression materials obtained by conventional DSC. Our pioneering TMDSC study [45] was designed to obtain fundamental information about impression materials and seek correlations with their relevant properties. 

Chloroprene rubber also generally offers good compression set properties. A higher percent of the compression set means a permanent deformation of the rubber matrix in a compressed form. The filled vulcanizates show a marked... 

Compression set A measure of permanent deformation remaining in an elastomer or flexible foam after a deforming force is removed. For most applications, a low degree of compression set is desirable. 

The presence of cross-links also helps in maintaining a desired geometry. Engineering thermoplastics, which contain no cross-hnks, are subject to creep, or cold flow, under load. They also have poor compression set resistance. Set is a permanent deformation that occurs under a load. Cross-linked elastomers can vary significantly in their set resistance. The choice of polymer, vulcanization system, and degree of cross-linking can profoundly affect set resistance (as well as many of the other properties of the vulcanizates). 

Compression set n. A permanent deformation remaining after release of a compressive stress. It is a property of interest in elastomers and cushioning materials, such as plastic foams. See, for example, ASTM tests D 395 and D 1565. Sometimes used to mean creep. 

When rubber is used as the sealing material it is its elastic properties which are particularly important, since the distorted rubber exerts a pressure on the contacting surface to maintain the seal. Unfortunately, no rubber is perfectly elastic and the stress in rubber decays or relaxes with time. This stress relaxation can be measured directly, or its existence can be implied through the measurement of permanent deformation acquired by the rubber when subjected to a constant strain for a given period of time. This property is known as the compression set or permanent set of the rubber. 

Permanent set measurements provide a means of assessing the longer-term stability of the crosslink structure under the influence of deforming forces, heat, oxidation, etc. This type of test may be carried out either in tension (tension set) or, more commonly, under compression (compression set) by application of fixed stress or fixed strain conditions applied for controlled time and temperature. Permanent deformation occurs as a result of a variety of processes (see below), which result in a net permanent deformation of the article. This is obviously undesirable as it leads to both changes in dimensions and, where sealing properties are required, a reduction or loss of the sealing forces. 

Deformation, permanent set The deformation remaining after a specimen has been stressed a prescribed amount in tension, compression, or shear for a specified time period and released for a specified time period. 

Some viscoelasticity results have been reported for bimodal PDMS [120], using a Rheovibron (an instrument for measuring the dynamic tensile moduli of polymers). Also, measurements have been made on permanent set for PDMS networks in compressive cyclic deformations [121]. There appeared to be less permanent set or "creep" in the case of the bimodal elastomers. This is consistent in a general way with some early results for polyurethane elastomers [122], Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. The number of cycles to failure was found to be approximately an order of magnitude higher for the bimodal networks, at the same modulus at 10% deformation [5] ... 

When polyurethane is subjected to a compressive force, it will deform. When the force is removed, the material will recover some of its original shape. The amount of the deformation that does not recover is known as the permanent set (see Figure 7.9). 

---